Easy How-to Fix for a Cracked Plaster Wall An Illustrated, Step-by-Step Guide to the Best Way to Re-attach Loose Plaster on Walls and Ceilings
Gypsum plaster walls are both the delight and despair of owners of heritage homes. It's easy to get jealous of your neighbor's drywall — the lack of cracking, the appearance of an absolutely flat, smooth wall, and the fact that he can hang a picture without rock boring tools. Your plaster walls are too thick for current door and window jambs, so every new door or window has to be ordered with a costly special jamb, or has to be modified on site.
A Glossary of Plaster & Drywall Terminology
Like all fields in the building trades, plastering and drywall have their own peculiar terminology.
Here is some common plaster and drywall terminology as used in the Mid-West. In your locality, there may be slight variations.
U.S. Gypsum originally colored its plasterboard blue to distinguish it from its other gypsum board products. Other manufacturers adopted the color scheme.
Brown Coat: The second coat of a three-coat plaster wall or ceiling system. The brown coat is the leveling coat applied over the first or "scratch" coat of gypsum plaster. It is typically about 3/8" thick.
Any unevenness left in the scratch coat is removed by the brown coat which leaves a flat, level surface for the final or "finish" coat. The brown coat is no longer brown in color but the name persists from the days when the hydrated lime second coat was darker than the final or white coat.
Color Coat: See finish coat.
Finish Coat: The final coat of a three-coat plaster wall or ceiling system. It is applied to the second or "brown" coat and is finished to a smooth surface ready for paint or wallpaper.
The final (finish) coat us usually hydrated lime plaster or a blend of gypsum and lime plaster because lime plaster works to a smoother finish than the stronger but coarser, gypsum plaster. It is also called the white coat (from the color of lime plaster which is whiter than gray or brown gypsum plaster under it), especially by old-time plasterers; and may also be called the color coat because chemical tints were often added to the lime plaster to give the wall its final color.
Finisher A person who tapes and finishes gypsum drywall. Rockers hang the drywall, finishers finish it. Rockers are usually not finishers and vice versa.
Greenboard or Green board: A type of gypsum drywall suited for use in damp (but not wet) areas such as bathrooms. U.S. Gypsum originally colored its water-resistant gypsum board green to distinguish it from its other gypsum board products. Other manufacturers adopted the color scheme. The newer purpleboard and paperless gypsum drywall have now replaced greenboard for applications where mold and water resistance is necessary.
Gypsum Board: A panel of gypsum drywall.
Gypsum Drywall: Panels made of a sandwich of gypsum plaster pressed between two sheets of paper. The panels are used to cover interior walls and ceilings and are the most common materials for that purpose since gypsum drywall replaced true plaster in the 11940s and '50s. It may also be used as exterior wall sheathing, in which case a water-resistant material replaces the paper. Synonyms include gypsum boards and drywall. Also erroneously called plasterboard.
Gypsum Plaster: A plaster mixture containing gypsum as the bonding agent. Discovered in the 18th century, gypsum replaced lime as the preferred bonding agent in plaster and cement because it is stronger than lime and cures much more quickly. Commonly used as the first and second coats of a plaster wall, with the smoother lime plaster used as the finish coat.
Horsehair Plaster: An early form of hydrated lime plaster to which horse hair was added for greater strength. Now rarely used except in historic restorations. It has been replaced by fiberglass in common use because fiberglass strands are stronger (and because horses have raised strenuous objections).
Lath: Thin wood strips or metal mesh attached to wall studs and ceiling joists to which plaster is applied.
Lather: (Pronounced "LATH er" with a hard "TH" — as in "baTH", not "lath er" with a soft "TH" like in shaving cream.)
A person who attaches lath to wall studs and ceiling joists. Once the lather is done, the plasterer applies plaster to the lath.
In earlier times, Lathers were essentially apprentice plasterers. Plasterers themselves seldom attached lath — too menial a job for a skilled plasterer. Today both lathing and plastering are usually done by the same person.
Lime Plaster: A plaster mixture containing hydrated lime as the bonding agent rather than the more common gypsum. Until the use of gypsum was developed as a bonding material in the 18th century, this was the only type of plaster available. It is common to use gypsum plaster as the undercoats in a plaster wall because gypsum plaster is stronger than lime plaster; with lime plaster as the final coat because of its smoother finish.
Plaster: A composition of hydrated lime or gypsum, sand, water, and sometimes hair or other fiber, applied as a thick paste to walls and ceilings and allowed to harden through a chemical process. The sand (or sometimes chalk) is the aggregate that gives body to the material, the lime or gypsum is the adhesive that bonds the sand particles together. The water is the carrier that makes the material workable.
Plaster is similar to concrete and stucco. For plastering a wall, gypsum mixes are commonly used for the first two (scratch and brown) coats of plaster and a lime mix for the final coat because it finishes to a smooth, hard surface.
In earlier times horsehair was used to strengthen the plaster and make it less subject to cracking. Today fiberglass strands may be used for the same purpose.
Although we routinely refer to lime plaster and gypsum plaster as distinguishable products, in practice most commercial plaster mixes contain both lime and gypsum. Lime plaster simply has more lime, and gypsum plaster more gypsum.
Plasterboard or Plasterboard; A gypsum board product used as an underlayment for wet plaster. Also known as plaster lath and rock lath.
The original substrate under wet plaster was wood lath — thin strips of wood attached to studs over which three-coats of plaster were applied. In the 1930s this was supplemented with expanded metal lath which was faster to install and bonded better to the plaster. In the 11000s plasterboard began to replace both wood and metal lath.
It was immediately popular because it substituted for the first, or "scratch" of plaster. It was not only faster and cheaper but made a better wall. Plasterboard was short-lived, however. It quickly morphed into gypsum drywall that needed no plaster coating at all. The surface of the board is also the wall's surface.
Plasterboard is made in solid, perforated, insulating and even acoustical versions.
The term plasterboard is commonly used, even by people who should know better, to refer to gypsum drywall but this is an incorrect use of the term that causes a lot of confusion.
Plaster Lath: See plasterboard.
Plasterer A person who applies plaster to a wall or ceiling. The plasterer follows the lather who attaches the lath to the wall studs. He or she, in turn, is followed by the finisher who applies the final or finish coat. May also refer to a person who applies stucco coatings to exterior walls. Today's plasterers are often called on to apply stucco, unlike prior times when stuccoers and plasterers were two different trades.
Rock Lath: See plasterboard.
Rocker: Slang term for a person who hangs gypsum drywall. From the brand name of the original gypsum board, "Sheetrock" by U.S. Gypsum. Hanging gypsum drywall is similarly referred to as rocking.
Scratch Coat: The first coat of a three-coat plaster wall or ceiling system. The scratch coat is the structural coat of the wall. It is forced into the gaps in the wall lath so that when it has cured it forms a strong mechanical bond with the wood structure of the wall. It is usually applied about 3/8" of an inch thick and scored with a scoring tool before it has cured. The scores or scratches help the second or "brown" coat grip the base coat. After 1950 plasterboard (or plaster lath) replaced the scratch coat in most residential walls and ceilings.
Sheetrock®: A trade name for gypsum board manufactured by United States Gypsum (USG). It has migrated into the vernacular and the "sheetrock" is commonly used to mean gypsum drywall. Sheetrock was the first widely accepted gypsum drywall product in the U.S., and "sheetrock" has been used to refer to gypsum board for so long that gypsum board installers are commonly referred to in the construction trades as rockers.
True Plaster: A term used to describe plaster for the purpose of distinguishing it from gypsum drywall.
Wallboard or Wall Board: A widely used slang term for gypsum drywall but also used to mean plasterboard. Unless you know the context, its use is usually ambiguous, and it should be avoided.
White Coat: See finish coat.
But, three-coat wet plaster walls are actually better walls. It's just that after repairing the umpty-umpth crack, the good things about plaster seem to get lost in the well of aggravation.
True plaster is unmatched in durability and strength. It's actually a form of concrete and very tough stuff. It resists fire better than gypsum drywall and greatly reduces sound transmission, which results in a much quieter house.
It is a hostile environment to mold and mildew because, unlike drywall, it has no paper facings. It is the paper that mold and mildew feed on. And, its high tensile strength makes it impervious to most damage caused by blunt force — being hit by the doorknob, for example. Drywall will dent readily, plaster shrugs it off with an impish grin.
Still, after the ump-teenth spider crack trying to hang a mirror, there is the temptation to just do away with the old plaster and replace it with nice, new, crackless, smooth drywall.
Our advice, after working with old houses for many years, is don't do it.
True gypsum plaster, with all its imperfections and evidence of age and use, is a part of the original fabric of your house. It is a major architectural element of the distinctive look, feel and character of your home. Its contours and slight imperfections are an enduring monument to some of America's early master craftsmen. So, if only for the practical reason that it makes a better wall, plaster walls and ceilings should be left in place and restored, if at all possible.
Besides, restoring plaster walls is almost always less costly than replacing them. And, we are huge fans of less costly. We are also big fans of the de minimus rule.
The De Minimus Rule Always do the least to your old house that will solve the problem.
Otherwise, you risk doing real harm to its essential character.
How Plaster Becomes Detached from the Wall
Until the 1930s, plaster walls were made with wood lath and lime or gypsum plaster.
Lime plaster dates from antiquity. It has been found on buildings from the Minoan culture on Crete and structures dating to 7200 B.C.E. at an archaeological site in Jordan. It is composed of hydrated lime, sand, water and some form of pozzolan material formed originally from volcanic ash. In more modern times it was usually reinforced with horse hair — much the irritation of a great many horses. Its chief drawback was its very slow curing time: often months.
Faster setting and stronger, gypsum plaster began taking over from lime plaster in the 19th century, and by 1900, most plaster was gypsum. Gypsum plaster is stronger than lime paster and sets in about 72 hours. It can be applied in thinner, lighter coats and greatly speeds construction.
During the 1930's wood lath gave way to metal lath and finally to plasterboard or "rock lath" which was used until the 1950s post-war housing boom when an entirely new wall system, gypsum drywall, replaced wet plaster walls in most localities. This is the wall system still in use in contemporary construction.
In some areas of the country, paster on rock lath is still the preferred wall — most notably in New England and the Mid-Atlantic states. Despite the claims of its admirers, it really has no advantages over skim coated gypsum drywall.)
Wood lath (or split lath) is just thin (1-1/2" x 1/4") strips of wood, usually pine, nailed horizontally over the wall studs about 1/4" apart. This gap allowed wet plaster to be pushed into the spaces between the lath strips where it slumps down on the inside of the lath.
When cured, the slumps form plaster "keys" that act a mechanical bond, tying the plaster to the wall framework. Since plaster does not actually stick to wood, these keys are the only connection between the wood-framed wall and its coating of plaster.
The base or "scratch" coat is scored to provide a rough surface so the next ("brown") coat will adhere. The brown coat is also scored but more lightly, to help the finish coat bond.
Once cured, wet plaster is a tough, durable, rigid material that, if left undisturbed, will last nearly forever. Unfortunately, things can happen to disturb plaster. The most common are settling and water.
When houses settle (and all houses do settle to some extent), the wood framework of the wall shifts, and if it shifts enough it can break off the keys holding the plaster to the wall.
Water can have a similar effect. Wood lath naturally swells when exposed to water. Seasonal changes in humidity are unlikely to be a problem but if the wall gets really wet, as from a leak, the wood may swell enough to break the keys.
If enough of the keys are broken, the plaster becomes detached from the lath.. Detached plaster tends to crack or even, if the detachment is over a large area, fall off the wall.
It is possible to have small cracks, especially around the tops of doors and windows, with minimal or no plaster detachment. But, if you have long cracks going across or up the wall, then most likely the plaster is detached behind the crack. If you have two or more roughly parallel cracks, then the plaster is probably also detached between the cracks.
Just patching over the crack is not the cure. It is, at most, a band-aid. As long as the plaster remains detached from the wall, the crack will inevitably reappear. Often just leaning on the wall is enough to expose the crack. For the repair to be permanent, the plaster has to first be re-attached to the wall.
Re-attaching Loose Plaster
Once keys are broken, they no longer mechanically fasten the plaster to the wall. We cannot recreate the keys, so some other form of reattachment is required.
There are two kinds of plaster reattachment: mechanical and adhesive.
Mechanical fastening involves the use of the old familiar remodeler's friend, the plaster washer or "plaster button". This is merely a thin washer, about the size of a quarter, designed for re-attaching plaster to wood lath.
They have been around for most of a century but are not a common hardware store item. We usually get ours from McFeely's but they are available online from several sources.
The traditional washer is galvanized steel, but we are seeing a lot of plastic washers these days. Plastic is not as strong, is thicker and harder to conceal, and tends to split. Best to use the traditional steel washer.
Their use is straightforward. A small hole (about 3/16") is drilled through the plaster but not through the lath. A galvanized flat-head screw is fitted to the washer and screwed into the wall through the lath.
This creates a new mechanical bond between the plaster and wood wall beneath the plaster.
Plaster washers work well but don't finish well. They stick out from the plaster about 3/16th of an inch (or more for plastic washers). This may not seem like much, but a 1/4" coat of plaster is needed to conceal a 3/16" projection — a fairly thick skim coat. This is not only a lot of work but often results in an uneven wall that looks patched.
More than a two dozen years ago, my partner and I were struggling around midnight to get the final coat of plaster on a wall repair in his old farmhouse near Pilger, Nebrasla. The plaster washers were making the job harder and much more involved and we were plenty annoyed.
We started talking about the idea that maybe we could glue the plaster to the lath and do away with plaster washers.
The next day we went to the Big Box lumber store to buy one tube of every kind of construction adhesive it sold and started gluing plaster chunks to wood. Amazingly, almost all of them worked — some better than others, of course.
The big problem was that most took a long time to set up hard. A few years later, the Henkel Corp., maker of Loctite adhesives came out with Powergrab® All-Purpose Construction Adhesive that bonded in a few hours rather than in two days. We had found the right glue.
Here's how it's done.
Just a Few Basic Tools are Required
You will need these tools:
- a drill with a 3/16" concrete or masonry bit,
- a caulk gun,
- a Phillips screwdriver (or Phillips bit for your drill),
- a (metal) putty knife,
- a set of drywall knives (6", and 12" knives at a minimum),
- a drywall mud tray or plaster hawk (tray is easier, hawk is faster),
- a painter's 5-in-1 tool,
- a sponge and bucket.
- a can of compressed air.
- a shop vac, and
- drop cloths (lots of them).
- a spray bottle,
- a sanding pole or block, and
- sanding screen or 120 grit open-coat sandpaper.
This safety equipment is required:
- a dust mask,
- a pair of safety goggles,
- rubber gloves.
Dust masks and rubber gloves are disposable and should not be reused. They are sold in boxes of 5 to 50 at your local hardware store. You will need the smaller box.
Rubber gloves are made from latex or nitrile. A lot of people are allergic to latex and don't know it. We recommend nitrile gloves. Be sure to get the slightly thicker gloves made for construction work and not the thinner examination gloves used in the medical profession.
No Exotic Materials are Needed
The materials you will need are readily obtained from local hardware stores.
- Powergrab® adhesive in a pint tube used with a caulk gun. One tube for every 50 square feet to be repaired.
- 1-1/4" wood-thread screws. We use drywall screws, cabinet screws or deck screws — whatever we happen to have lying around. You will need approximately 3 screws for every square foot of plaster to be repaired. These and the fender washers will be used to clamp the wall while the glue dries, then will be removed so you can use them on something else later.
- Fender washers. One 2" or 3" fender washer with a 1/4" hole for each screw.
- Drywall top coat joint compound.
- DuraBond setting joint compound (maybe),
- Plaster of Paris.
- Windshield washer fluid (to remove wallpaper. (Forget it if you don't have to remove wallpaper.).
Old paint may contain lead
Lead is considered toxic by the EPA which has published guidelines for containing lead in home repairs and remodeling. Safety from lead paint consists primarily of keeping paint particles out of the air where they can be inhaled.
- Always wear a dust mask and rubber gloves, keep the paint damp with a spray bottle so the lead does not form dust in the air. Use a disposable plastic drop cloth.
- When you are done, roll up the drop cloth so the particles do not escape, and put it into a plastic bag along with your gloves and mask for disposal. Securely tie the neck of the bag. Once contained, the lead can be disposed of as ordinary trash.
To learn more about safely handling lead paint, read the EPA Lead Renovation, Repair, and Painting Program Rules.
Take care using the 5-in-1 tool
Scraping on plaster with a 5-in-1 tool is very much like honing it on a barber's strop. It can eventually get razor sharp.
The Process: Step by Step
For best results, read through the entire process at least once before you start, then follow the steps closely. Any questions? Contact us.
Step 1 Remove Loose Paint & Wallpaper
Check the wall for loose paint or wallpaper. All of this has to come off. Trying to bind plaster to loose paint or paper guarantees it will eventually loosen.
The most effective tool we have found for removing multiple layers of loose paint and wallpaper is the painter's 5-in-1 tool. Slide it under the loose paint or paper and lift it off.
If the final layer is paint, then you don't have to remove all the paint, just the loose bits.
If the final layer is paper, then all the paper has to come off. Otherwise, it will absorb water from any plaster applied over it and loosen — making a soggy mess of wet paper and plaster that will probably end up on the floor.
You can rent a wallpaper steamer from just about any tool rental store or your local wallpaper shop. We have found, however, that ordinary windshield washer fluid applied as a spray works nearly as well, and is very cheap — about $3.00 per gallon. Wet the paper with the fluid until it appears very damp, wait a few minutes for it to soak in, and scrape the wallpaper off with your 5-in-1 tool or a stiff putty knife. If there are many layers of wallpaper, more than one application may be required.
Wall Treatment Archaeology
Removing paint and wallpaper is never a cheerful task but it does give you a chance to do some wall treatment archaeology.
Our wall showed only three layers of paint, very unusual for a 90-year-old house. The original color was olive green. The next layer was tan. Both of these were oil-based indicating that the second layer was probably applied in the 1950s. The final, visible layer of paint was latex in white.
Our customer elected to return to the olive green paint and arranged her color scheme around that original color. So, we saved some chips for Sherwin-Williams to color-match. It matched closely to Ruskin Room Green.
The cabinets were pre-war and original to the house — unusual since most Arts & Crafts kitchens were "updated" in the immediate post-war period. (See Arts & Crafts Styles: Craftsman, Prairie and Four-Square Architecture for more on Arts & Crafts kitchens and baths). The original cabinet color was bright red. This was painted over in yellow, then white.
With olive walls and red cabinets, this was a bright, colorful, cheerful kitchen in 1928. And, we hope whoever lived in it then enjoyed it a lot.
Step 2 Mark the Boundaries
To make sure you reattach all of the loose plaster, you need to find and mark the perimeter of the detached area. The presence of a crack indicates loose plaster but not necessarily the full extent of the detached area.
There are two sure ways to find loose plaster. One is to push on the plaster. If it is loose, it will often move and feel spongy, but not always. The other is to rap on the wall with a knuckle. Detached plaster hounds hollow. Solid plaster sounds, well, solid. Using the "push and look" and "tap and listen" techniques, find the edges of the detached area, and mark them with a pencil line. You don't have to be exact. Close is good enough.
Step 3 Drill a Lot of Holes
We need to get glue behind the plaster, between the plaster and lath. To do this we need to drill 3/16" holes using a masonry bit 1/2" from any crack, on both sides of the crack, spaced every 3-4" apart and every 8-10" in the area between cracks.
This process requires a little bit of a "touch".
You want to drill through the plaster to but not through the lath. Figure the plaster to be about 1/2" to 3/4" thick. So, put tape on your drill bit at the appropriate depth. Drill slowly until the drill starts feeling springy. This means you have probably hit the lath, so stop. If there are little bits of wood on your drill bit, then you have gone far enough.
To test the hole, push an awl, small Phillips screwdriver, or piece of wire clothes hanger into the hole. If the bottom of the hole pushes in just a little then springs back, you have hit lath. If it does not push in or grinds when you wiggle the probe, you are still in the plaster.
If you accidentally drill all the way through the lath, no problem. Just move your drill bit about 1/2" right or left of the original hole and drill a new hole. Mark the bad hole by circling it with a pencil. This means, "Don't glue up this hole". If you hit the open space between lath strips, do the same thing. Move 1/2", this time up or down, and drill a new hole. Mark the bad hole.
Step 4 Clean Out the Dust
Drilling left a lot of dust and some small plaster particles in the holes. Particles may prevent the plaster from seating to the wood lath and dust interferes with the adhesion of the glue, so we need to get rid of both.
Vacuum out each hole with a shop vac then blow out the hole with compressed air — make sure you are wearing your safety goggles. Finally, vacuum each hole again to be sure you got everything. Compressed air in a can is sold in almost every hardware or electronics store.
Rarely, but sometimes there is debris caught at the bottom of a pushed out section of plaster. This will keep the wall from pulling in flat unless it is removed. To remove it, drill a series of 1/4" holes along the bottom of the detached area spaced about 1/2" apart. Vacuum out the debris.
It helps to put gentle pressure on the piece of lath with an awl or screwdriver to help release any particles that may be wedged in. We follow vacuuming with compressed air, then vacuum again to make sure we got everything. If some debris is still stubbornly wedged, a wire coat hanger with a 1/2" right angle hook at one end can be used to dislodge any stubborn particles.
Step 5 Insert the Glue
We use a Grainger® dripless caulk gun, which is definitely a pro tool, and expensive as caulk guns go. It works so well, however, that we think it's worth the outlay. If you don't have one, keep some paper towels around to mop up dribbles.
Insert the nozzle of the tube in a hole and press down so the nozzle seals the hole tightly. Give it about 1/2 of a full trigger pull's worth of glue — slowly. You will probably feel a little resistance as the relatively thick adhesive works its way into the narrow space between the plaster and lath. Release the pressure on the gun and remove it from the hole.
Glue each hole the same way, avoiding the bad holes you circled with a pencil mark. The glue may drip a little out of the hole, which is where your paper towels come in handy.
If the glue goes in too easily, then you have probably drilled through the lath and the glue is just going into air. If it blows back right away, then your hole is not all the way through the plaster. Mark this hole with a penciled "X" and move onto the next hole. Come back later and redrill, then glue the "X" hole.
If you are not sure you are getting glue into the area between the plaster and lath, here's a simple test. Drill another hole 1/2" to the side of the hole to be glued. Keep adding glue until it squeezes out of the test hole. You can do this as many times as you like until you get the sense of what a properly glued hole feels like.
If your hole is all the way through the plaster but you can't force glue into the hole, then the lath is probably pressed hard against the plaster. To create a little space for the glue, drill a hole about 1/2" to the right or left of the glue hole, and using a screwdriver or awl, push on the lath gently while adding glue. This should give the glue enough room to wriggle between the lath and plaster.
There is a time limit to this process. Powergrab® begins to set after about 15 minutes. So only glue up as many holes as you can glue and screw in 15 minutes. For an experienced operator, this is about 60 holes. If this is your first time, figure on about 25-35.
Step 6 Clamp the Plaster
Now we need to clamp the loose plaster to the wood lath while the glue bonds. This is done with screws and washers. You can use plaster washers. We did for a few years but fender washers work better, and they are more widely available at just about any hardware store or your local body shop supplier.
You will need 2" or 3" washers with 3/16" or 1/4" holes. The screws should be no longer than 1-1/4". Any longer and there is a risk of hitting power lines or pipes inside the wall — things you do not want to hit with a metal screw.
We use deck screws or drywall screws — whatever we have on hand. They won't be there long, and we can always reuse them for something else later.
Insert one screw into a washer then into each hole. Tighten it down until the screw is fully seated. You will see the plaster draw into the wall as the screw is tightened.
Start at the outside and work toward the middle. This draws the plaster in gradually, reducing the risk of additional cracking.
How to Keep Washers from Sticking
A Reader Tip from
Apply thin brown package shipping tape to the side of the washer that presses against the wall. The PowerGrab won't stick to it and the washers are ready to reuse for the next section.
I lay the tape sticky side up on a flat surface and drop the washers on. Then cut into strips and fold the excess tape over the top.
If the plaster is very loose and more than 1/4" away from the lath, tighten each screw just part way until all the screws are started, then go back and tighten the screws all the way down. This helps prevent further stress cracking in the plaster by moving it gradually and evenly rather than all at once.
When all the screws are set, wash the wall with a sponge and water to remove any excess glue, clean up and call it a day.
Tomorrow you will remove the screws and washers. Odds are pretty good (unless you use David Gabbe's shipping tape trick) that the washers will be stuck to the wall — Powergrab® glues just about anything to anything else. Use your 5-in-1 tool to pry them loose. Insert the edge of the tool under the washer and thump it with your hand. The washer will pop off. If a little paint and plaster come off with it, no harm is done. We will fix this in Step 8.
It happens very rarely but if some of the plaster still seems a little spongy, no worries. Just drill new holes in the springy areas, add glue and screw down overnight. You can do this as many times as you need to until all the sponginess is gone to your satisfaction.
Step 7 Level any High Spots
Not often but sometimes, in a badly delaminated wall, not every piece of loose plaster will be fully seated against the lath. Some plaster bits may have come loose and lodged between the lath and plaster preventing the plaster from fully reaching the lath in some spots. And, sometimes the lath itself is deformed or detached or a protruding nail prevents the plaster from seating fully.
There may also be areas where someone in the past has badly patched the plaster. This wall had both problems. In badly degraded areas, the plaster did not draw back perfectly flat, and there was one ham-handed patch job by someone who must have used a butter knife as his main plastering tool. Anyway, it was very rough looking.
A plasterer will generally try to fill such defect with new plaster, or break out the patch and start over. We want to do neither because we don't want the unevenness of a heavy mud-over, and we don't want to risk damaging the wall by breaking out the patch. What we do want to do is smooth down the rough areas to make the wall ready for our final plaster coat. The best way to do this is with a belt sander and a 40-grit belt.
This is messy and creates a lot of dust — a lot of dust — so make sure everything in the room is covered, the shop vac is attached to the sander, and you are wearing an approved dust mask and safety goggles. If you wear eyeglasses, do not rely on the glasses alone for eye protection. Wear goggles.
Step 8 Fill the Holes
The plaster is now firmly re-attached to the lath, and the high spots have been leveled out. It is now time to start rebuilding the surface of the plaster. If you are not a plasterer, with all the wonderful but arcane, plastering tools in your toolbox, no problem. Typical drywall tools will work just as well, and you are more likely to own them.
Our first job is to patch all the holes we drilled and all the divots left when washers were removed. You can do this with drywall joint compound and wait the two or more days it takes to air dry. But we like to move a little faster than that, so we use plaster of Paris.
Plaster of Paris is a setting plaster with a myriad of uses including making castings and splinting broken arms and legs, and, in our case, filling holes and divots. It has a very short working time, so mix just as much as you can apply in 15 minutes or so — about one pint. Using a putty knife, press the plaster into each hole and then smooth it out.
Mix the plaster in a clean plastic cup. Do not mix a new batch in a cup that already has old plaster in it. It will vastly speed up setting time, giving you little time to work.
Wait at least 45 minutes for it to set before proceeding to the next step.
Step 9 Patch the Cracks
We have re-attached the loose plaster to the lath, and this will keep cracks from getting any worse or new cracks forming. Now we have to bond the cracked pieces of plaster back together.
We have read a number of articles and seen a lot of videos in which the cracks are just plastered over with joint compound. This does not work. Even though the plaster is now firmly attached, just plastering over the cracks is a virtual guarantee of future hairline cracking.
What's needed is a firm bond that joins the two side of the crack together permanently so they cannot separate in the future. This requires nylon mesh tape for the structural integrity needed and joint compound to bind the ape to the wall. Joint compound alone will not do the job.
Use pieces of tape short enough to follow the twists and turns of the crack, keeping the crack roughly in the center of the tape. Apply the tape to a vertical crack starting at the bottom and working up. Since most of us tend to apply plaster top to bottom, this prevents the taping knife from catching an edge of the tape and pulling it off the wall. If the crack is horizontal, apply left to right for a right-hander, right to left for a left-hander.
Some mesh tape is self-sticking, some not. If yours is not, you will need to apply a thin coat of drywall joint compound over the crack before embedding the tape in the mud with your taping knife. Some of our plaster guys think all tape should be embedded, even self-sticking tape because it creates a stronger bond. Others think it works just as well to stick the tape to the wall and mud over it — and it takes less time and less work. Personally, I am from the "less work" school.
Adhesive mesh tape is easy to apply. Roll out the length you need, smooth it down with your 6" knife. To cut it, press against the tape with your knife, and tear it. The knife is sharp enough to easily cut the tape. For a good video on applying mesh tape, go here.
Mesh tape is never used on inside corners. For inside corners, you need a material that will crease sharply, and mesh tape will not.
Some pros will tell you to never overlap mesh tape but we do it all the time — just don't build up multiple layers of tape. Two layers is the max.
The Window Screen Trick
A Reader Tip from
If you have a lot of cracks close together, instead of taping each crack individually, you can reinforce the whole area with fiberglass cloth. This is just a very wide version of fiberglass tape — up to 36" wide compared to the tape's 4" width.
We use it sometimes but most of the time, we just use nylon insect screen — the stuff you put on screen doors. A technique introduced to us by plasterer extraodinaire, Mitchell Rōem. It works just as well at about 1/3rd the cost.
Window screen comes in 100' rolls but any hardware store will cut just the length you need. Be sure to get the coarse rather than the fine screen and make sure it is nylon, not aluminum or steel.
Since insect screen is not self-adhesive, you will need to embed it in joint compound. Apply a thin (not more than 1/4") coat of compound the size of the screen plus a little extra. Lay the screen over the compound, pressing it in slightly in several places to hold it in place. Then, starting at the middle with you 6" knife, embed it evenly in the compound. Scrape the excess compound from your knife back into the mud pan or hawk after each pass.
Before nylon and fiberglass were invented, canvas or burlap cloth was used, and it is still an option. but these are actually more expensive these days than either nylon screen or fiberglass cloth, and harder to work with. (For more on "canvassing" a plaster wall, see Preservation Brief 21: Repairing Historic Flat Plaster Walls and Ceilings from the National Park Service.)
Regardless of what method you use, apply drywall joint compound over the tape, tapering the compound on both sides of the tape using an 8" drywall knife (if you don't have one, a 6" knife works nearly as well). This will give you a taper of about 4-5" on each side of the tape, which is plenty. If you leave a ridge or two, don't worry about it. We will scrape these off in the next step.
If your wall has missing plaster (the wood lath is exposed), now is the time to patch the hole. Use DuraBond® or any similar setting-type drywall compound. Apply a thick coat to the wood lath, pushing it into the spaces between the lath strips. This will form the keys that lock the new plaster to the lath. Don't fill right to the surface of the wall, keep it recessed about 1/4"
Before it sets rock hard, using a scoring trowel (or if you don't have a scoring trowel, any tool that will score the soft plaster, a screwdriver or the point on your 5-in-1 tool, for instance) to score the plaster. Score in parallel lines about 1/2" apart. The ridges left by scoring are what the next coat will bind to.
After the plaster has set, about 1 hour for DuraBond, apply the final coat, finishing it off even with the rest of the wall. If you are unhappy with the final result, give the plaster a chance to set almost hard, then scrape away any ridges and high spots. Give it about another hour to harden up, then apply a very thin coat of regular (non-setting) drywall compound. Don't panic if it is still not perfectly smooth. We will even it out with the final skim coats of plaster.
Now clean up and let everything set up for about 24 hours. A fan in the room speeds up the process.
Step 10 Apply the Finish Coats
A seasoned plastering professional would probably apply one perfect finish coat, accept your generous check, and thank you for the work. But, since were are not that accomplished, we are going to apply two thinner skim coats.
First, scrape down any ridges you left when taping the cracks. Use your 6" taping knife or 5-in-1 tool. Drywall joint compound scrapes off easily. Hold the knife at a low angle to the wall and used the edge like a paint scraper. If you accidentally dig into the wall here and there, no matter, we'll fix it in the next step — but try not to do it.
You have several options for the final, skim coat of plaster. You can use actual gypsum/lime top coat plaster. It is still available from U.S. Gypsum and National Gypsum. You can usually buy it from your local drywall supplier, although he may spend a long time looking for it in the warehouse. We like Diamond Veneer Finish by USG but they make several top coat plasters, all of which will work.
A modern substitute is DuraBond from the same company. Durabond is a setting joint compound intended for drywall but it takes a nice finish and cures much faster than true plaster.
Our choice for this job, however, is ordinary topcoat joint compound. It is not as hard as true plaster (but we will use a trick to make it nearly as hard). But, it is very forgiving and much easier to work with, especially if you have not done a lot of plastering.
Some of our plasterers prefer to thin the joint compound a little by mixing it with, at most, one or two cups of water per 5-gallon bucket of joint compound. It does make the compound go on thinner but it is also harder to handle. If you are new at the game, we recommend against it.
Using the widest drywall compound knife you can handle (we use a 12" knife, some hearty souls use a 14" knife) start at the top of the wall and apply a thin layer (about 1/8") of drywall compound to the wall. Immediately skim the excess mud from the surface and return it to your mud pan or hawk. Hold the knife at about a 30° angle and press down fairly hard. The final result should be a thin, 1/16" or so, coat.
Try to get as smooth a wall as you can. But, if you get a few tool ridges, don't worry, we will knock those down later. Keep your knife clean by scraping it frequently on the edge of the mud pan or hawk. "A clean knife for a clean job" — something apprentice mudders hear over and over and over…
If the section to be skimmed is large — most of a wall, for example, work in manageable swaths. Start at the ceiling and skim down vertically to just above the middle of the wall. Then work horizontally in about three-foot sections across the wall. Finally, skim from the bottom of the wall up to overlap the middle section you just finished. These are the easiest motions for most people to make without excessive fatigue.
When you finish this coat, clean up, wash your tools and wait 24 hours before applying the final coat.
Start your final coat by first scraping down any ridges in your first coat with your drywall knife. Get the surface a smooth as you can. If there are a lot of ripples, bumps or other protrusions, you might need to use a sanding block or pole sander to go over it lightly. "How to Sand Joint Compound" below for the correct sanding technique.
The second coat is a repeat of the first and applied in the same way but thinner. It just fills in any low spots left by the first coat, so you don't need it very thick. Put in on very, very thin — about the thickness of a piece of paper. The thin coat is will not leave any knife ridges. If it does, your coat is too thick. Put more pressure on the knife to thin the coat.
Here you might consider thinning the drywall compound with just a little water to make it easier to apply a very thin coat. Don't get it too thin. If you do it won't stick to your knife. You want it about the consistency of cake icing. If you don't know how thick that is, ask your better half to supervise.
The final result should look a lot like the picture at right (or above if you are on a smart phone or small tablet). If you cannot get this result with two coats, let the second coat cure, then go over it again with a very, very thin third coat.
Step 11 Smooth the Plaster
If you elected to use one of the true plaster top coats, you can't sand it. It will finish as hard as a rock and any sanding will simply disturb that hard-skin that forms on the surface of the plaster, and you will have to patch it.
But, we prefer joint compound, and joint compound needs smoothing of some kind. The choices are sanding or burnishing. Let the top coat is completely dry — about 24 hours — less if you help it along with electric fans.
How to Sand Joint Compound Like a Pro
If you decide to sand, use a sanding screen (preferred, and available at any well-stocked paint store) or 120-grit open-coat sandpaper and a sanding block or pole sander. A sanding sponge can be used in place of a block but it's more work. Sanding drywall compound is not difficult but it is different from sanding wood. Here are the rules:
1. Less sanding is better than more sanding. Lightly sand the entire patched area. Those new to plastering often try to get the wall perfectly flat and even by sanding and more sanding. You won't, so don't try it. All you will do is cut through the skim coat and have to patch it. Just knock down any projections and get the surface reasonably flat and smooth.
2. Protect yourself and your house from dust. Wear a mask and cover anything you do not want to get dusty. Vacuum up the dust when you are done. To reduce drywall dust, you can rent or rent or buy a sander in most localities that attaches directly to your HEPA shop vac. Most ordinary (non-HEPA) vacuums just shoot the dust back into the air.
3. Damp wipe the sanded area. Wipe the whole area with a damp cloth or sponge. Damp means "almost dry". You want a little water to help pick up the dust but not enough to dampen the wall finish. Rinse the sponge frequently. If water does not do it, try 408 cleaner. It is the only cleaner we have found that actually picks up drywall dust rather than just spreading it around.
The result should be a smooth but dull finish wall. It should not have obvious projections or depressions. If it does, do a little more sanding on the projections and fill in the depressions, wait for the compound to dry, and re-sand the filled areas.
Burnishing For that "Wet Plaster" Look
The other finish is burnishing. We like burnishing because,
1. it raises no dust and, therefore, makes less of a mess, and
2. in texture and appearance it is virtually indistinguishable from true plaster.
Burnishing is a very simple technique but it is work, so break out the Bengay for your soon to be sore muscles.
You may not get it all done at once. Since you can burnish any time before you paint or paper the wall, there is no hurry about it.
You will need a sprayer full of tap water and a 5" or 6" taping knife. If you are applying a decorative finish coat (see "Burnishing as Decorative Wall Treatment", this page), use a stainless steel knife to avoid leaving rust marks on the wall. Working about 4 square feet at a time, spray the top coat until it is damp but no water is flowing. Holding the knife at a very low angle, almost flat against the wall, run it across the damp surface several times in different directions. Use short, very firm strokes. We have a pattern we use for consistent results: top to bottom, then side to side, then both diagonals, then top to bottom again to finish off.
Burnishing compresses the top coat making it denser, harder and smoother, and gives it a little sheen.
The difference between burnished and unburnished joint compound is obvious to the touch. Rub your hand over a burnished area. It will feel hard, slick, and very smooth, just like the gypsum/lime final coat used in true wet plastering. Run it over an un-burnished area for comparison. The raw top-coat will feel rougher and powdery, not nearly as smooth or hard.
A burnished surface looks and feels so much like lime plaster that we have fooled professional plasterers into thinking it was indeed lime plaster.
Now, You are Done
The wall is ready for your wall finish: paint, wallpaper or whatever. Keep in mind that your burnished wall may look like real plaster but it is actually just drywall joint mud, so, if you paint buy the primer for drywall, not plaster.
Congratulations are due. You have just rescued a heritage plaster wall using a plaster-saving technique that works nearly every time but that few know about.
Your wall is now as good as new but just to be sure, you should inspect it periodically for further cracking — once every 50 years or so should be plenty.
You may want to leave the wall unpainted for several weeks. This allows it to dry completely and provides a reasonable interval during which you can, graciously and modestly, receive richly-earned accolades from the spouse, kids, neighbors, in-laws, the cable guy, etc.
Want to learn more about traditional plaster walls and how to restore them? Here are some resources.
• Repairing Historic Flat Plaster Walls and Ceilings, Preservation Brief 21, Technical Preservation Services, National Park Service. The National Park Service, which takes care of most of the Nation's public historic buildings, recommends retaining historic plaster if at all possible. Plaster is a significant part of the "fabric" of the building. Much of the building's history is documented in the layers of paint and paper found covering old plaster. For buildings with decorative painting, conservation of historic flat plaster is even more important.
• This Old House: How to Fix Damaged Plaster. General Contractor Tom Silva uses the Big Wally's "Plaster Magic" system to repair a plaster wall. Illustrated step by step instructions.
• Hodgson, Frederick Thomas. Plaster and Plastering: Mortars and Cements, How to Make and How to Use. New York: The Industrial Publication Company, 1901
• Leeke, John. "Saving Irreplaceable Plaster." Old House Journal. Vol. XV, No. 6, November/December 1987, pp. 51-55.
• Poore, Patricia. "The Basics of Plaster Repair." Old House Journal, Vol. 16, No. 2, March/April 1988, pp. 29-35. (Available online from Google Books, courtesy the editors of Old House Journal and Allphabet, Inc.)
• Stagg, W. D. and B. Pegg. Plastering: A Craftsman's Encyclopedia. Woodstock, New York: Beekman Publishers, 1976.