Glossary of Plaster/Drywall Terminology
There is probably no field in remo­deling in which the terminology used is more confused or confusing. To help alleviate some of the confusion, here are the actual names of plaster and drywall components, and materials.

Blueboard or Blue board: A term commonly used in the construction trades to mean plaster board. U.S. Gypsum originally colored its plaster board 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.

Drywall: See gypsum drywall.

Finish Coat: The final coat of a thee 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 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, finishes 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: See 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 1940s and '50s. Synonyms include: gypsum board, drywall. Also erroneously called plaster board.

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. 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, because fiberglass strands are stronger, and because horses strenuously object.

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", 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, and plasterers themselves seldom attached lath — too menial a job for a skilled plasterer. Today both lathing and plastering are probably 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 under coats in a plaster wall, because gypsum plaster is stronger than lime plaster; with lime plaster as the final coat because of its smoother finish.

Mudder Another name for a plasterer or drywall finisher, but only if you are close friends.

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. In both composition and purpose, plaster is similar to cements and stuccos. 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 or skimcoat 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, most commercial plaster mixes contain both lime and gypsum. Lime plaster simply has more lime, and gypsum plaster more gypsum.

Plaster­board or Plaster Board; A gypsum board product used as an under­layment for wet plaster. Also known as plaster lath and rock lath. Originally the 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, stronger and bonded better to the plaster. In the 1940s plaster board 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. Plaster board was short-lived, however. It quickly morphed into gypsum drywall that needed no plaster coating at all. The surface of the board was the wall surface.

The term plaster board 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: Another name for plaster board.

Plasterer A person who applies plaster to a wall or ceiling. The plasterer follows the lather who attaches the lath to the wall studs. 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 stucco-men and plasterers were two different trades.

Purpleboard or Purple Board: A type of gypsum drywall resistant to water and mold for use in damp (but not wet) areas. It has largely replaced green board for use in bathrooms and other damp areas.

Rock Lath: Another name for plaster board.

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 halp 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), used commonly 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 plaster board. Unless you know the context, its use is usually ambiguous, and it should be avoided.

Wet Plaster: A term used to describe plaster for the purpose of distinguishing it from gypsum drywall.


Gypsum Drywall That Looks Like Plaster
You may not be aware of it, but drywall finishing is described in levels. The Gypsum Association identified levels of finishing in its 1990 standard, GA-214-90: Recommended Specifications: Levels of Gypsum Board Finish. The standard was created to "precisely describe" the desired finish of walls and ceilings. ASTM (formerly known as the American Society for Testing and Materials, and now as ASTM International) has also published a similar standard as ASTM C 840-04.

There are six official levels of drywall finishing, to which we have added a seventh in our standards and specifications. The seventh, Level 6, produces a gypsum drywall finish that resembles a lime plaster wall in both hardness and sheen.

Level 6 Drywall Finish

Level 0: Unfinished. No finish of any kind, no taping. Used only for temporary walls.

Level 1: Taped. Paper tape is embedded in joint compound and smoothed with a taping knife or self-adhesive mesh tape is applied without any joint compound. The tape remains exposed and the heads of fasteners are not coated. This finish is used where appearance is unimportant. The surface is left free of excess joint compound but ridges and tool marks are acceptable. This level is often specified in the plenum area above drop ceiling ceilings, in attics, and service corridors, but in residences is rarely used. A level 2 finish is used instead.

Level 2: Embedded Tape. This level of finish is used where appearance is unimportant, but fire protection is required, as in garages; and as backer for tile and wood paneling. Colloquially this is known in the trades as "fire taping" or "tile taping". Joints and interior angles are covered with tape embedded in joint compound and wiped with a joint knife leaving a thin coating of joint compound over all joints and interior angles. Joint compound is applied over all fastener heads and beads. The surface is left free of excess joint compound but ridges and tool marks are acceptable. The usual tool is a 5" or 6" taping knife. A professional is likely to use a taping machine or "bazooka" and a wipedown knife which is essentially a taping knife with a long handle.

Level 3: 1st Finish Coat. Where the wall or ceiling covering to be applied is a bold texture or heavy wallpaper, this level of finish is usually sufficient. This is typically the finish applied to ceilings that are to be textured. It starts with a Level 2 finish. Any ridges or tool marks are knocked down, then one additional coat of joint compound is applied to all joints and fastener heads. The finished compound should be smooth and free from tool marks and ridges. If it is to be the final coat, it is sometimes sanded, but does not have to be and many finishers don't sand because of the risk of exposing the tape. The usual tool is an 8" or 10" drywall knife.

Level 4: 2nd Finish Coat. This is the normal wall finish, but it is also used for ceilings that are to be only lightly textured. An additional coat of joint compound is applied over a Level 3 finish, including all fastener heads, then the entire wall or ceiling is sanded smooth. This is usually an satisfactory finish if the wall is to be painted with flat paint or papered with light-weight wallpaper. If there is strong grazing light, minor dips and bumps in the finish will show, and it is not suitable if semi-gloss or gloss paint is to be used as these imperfections will become visible. The usual tool is an 12" or 14" drywall knife.

Level 5: Skim Coat. If a surface is subjected to grazing light, or semi-glass or glass paint is to be used, then this is the level of drywall finish desired. It starts with a Level 4 finish, then a coat of thinned joint compound is applied over the entire surface. It is commonly applied with a roller, then towelled off. This fills in any dips and leaves a smooth, uniform surface that minimizes the possibility of joints or fasteners showing through. If a ceiling is to be smooth rather than textured, this is the finish level desired because the grazing light of ceiling fixtures can reveal every dip and bump left by a Level 4 finish. The usual tool is an 12" or 14" drywall knife, but some professionals prefer an even larger knife, up to 18". Keep in mind that the larger the knife, the more difficult it is to handle and the longer it takes to become proficient.

Level 6: Burnishing. This is our added level of finish. It starts with a Level 5 finish, then the entire wall is burnished with a drywall knife to compress the finish and give it a plaster-like hardness and sheen. The technique is described in the main article. The usual tool is a 5" or 6" taping knife.

Burnishing as Decorative Wall Treatment


Burnished Wall Finish Rather than paint or wall­paper your new wall, you can use bur­nished plaster as the finish coat by adding tint to the skim coats.

From before the turn of the 20th century, up to the 2nd World War, architects often applied wall color to the final coats of plaster rather than paint the wall. There were several reasons. First, it often took plaster as much as six months to cure enough so paint could be applied. It also helped disguise chips and minor damage to the wall because the white plaster would not show through the finish color.

Today this kind of finish is often called Venetian Plaster, and plasterers and painters charge a lot of money to do it. But it actually nothing more than skim coating and burnishing with tinted rather than plain plaster. You can get plaster tints at your local drywall store. Add the tint to the drywall joint compound until you get the color you want. Remember, wet tint looks darker than dry tint, so test it on the wall in an out of the way area, and let it dry to see its final wall color.


stopsign Plaster pigment is toxic. Do not ingest or inhale the powder. Wear a face mask and latex gloves when handling pigment powders.


For added gloss, and moisture protection, a final coat of wax can be applied to the burnished wall. We use a good quality carnauba wax, but waxes especially formulated for plaster are available. Wait at least 24 hours before applying the wax. Not waiting may cause streaking on the final surface due to residual water in the joint compound. Waxing is recommended in areas exposed to continuous moisture such as bathrooms and kitchens.

How to Fix Loose Plaster

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 built out on site.

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 will not.

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 evidences 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 homer. 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:
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. Gypsum plaster began taking over from the older lime plaster in the 19th century, and by 1900, most plaster was gypsum. During the 1930's wood lath gave way to metal lath and finally to plaster board or "rock lath" which was used until the 1950s post-war housing boom when gypsum drywall replaced plaster walls in most localities (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).

Wood lath is just thin (1-1/2" x 1/4") strips of wood 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 wall. 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 bond 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 tends to 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.

When they keys are broken, the plaster is detached from the wall. 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 along the crack. If you have two 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 reattached to the wall.

Reattaching 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, mechanical and adhesive.

Mechanical Attachment


Mechanical fastening involves the use of the old familiar remodeler's friend, the plaster washer or "plaster button". This is merely a thin metal washer, about the size of a quarter, designed for reattaching 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 on line from several sources.

Their use is straightforward. A small hole (about 3/16") is drilled through the plaster, but not through the lath. A 1-1/4" or 1-1/2" galvanized (plaster is corrosive) 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 1/16" to 1/8"", so gobs of plaster must be applied to the wall to conceal the washers. This is not only a lot of work, but usually results in an uneven wall that looks patched.

Adhesive Attachment


More than a 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 in Pilger, NE. 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 in Norfork, NE to buy one tube of every kind of construction adhesive they had 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 overnight rather than in 24-48 hours. We had found the right glue.

Here's how it’s done.
 

How to Repair Loose Plaster


Just a Few Basic Tools are Required


You will need the following tools: You may also need these additional tools:

No Exotic Materials are Needed


The materials you will need are easily obtained from local hardware stores.

Step 1

Remove Loose Paint and 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 is guarantee it will eventually loosen. The most ef­fec­tive tool for removing multiple layers of loose paint and wallpaper is probably the painter's 5-in-1 tool. Slide it under the loose paint or paper and scrape it off.

Stop sign
  1. Be careful with 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 will eventually get razor sharp.
  2. Old paint may contain lead. Lead is considered toxic by the EPA which has published guidelines for containing lead in home repairs and remodeling. Always wear a dust mask, keep the paint damp with a spray bottle so the lead does not form dust in the air, and 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 for disposal, securely tying the neck of the bag. Once contained, the lead can be disposed of as ordinary trash.
If the final layer is paint, then you don't have to remove all the paint and paper, 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 real mess of soggy paper and wet plaster. 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.

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 save some chips for Sherwin-Williams to color-match. It matched perfectly 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 panted 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 of the Loose Plaster Area



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 move and feel spongy. 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 "rap 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 Lots of Holes



We need to get glue behind the plaster, between the plaster and lath. To do this we need to drill 3/16" or 1/4" holes using a masonry bit every 4-6" on both sides of any crack 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.

Stop sign Wear a dust mask. You don't want to breath in plaster particles or, worse, old lead paint.

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. 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.

Step 5

Apply the Glue



Powergrab® comes in a one pint (small) tube for use in a caulk gun. It may come in the larger, contractors' size quart tube, but we have never seen it. You will need one tube for each 50 square feet of wall. This project was 65 square feet, so we pulled two tubes out of supply. Cut the tip square so it is slightly smaller than your holes. You want it to fit inside the holes you drilled so it creates a good seal, but you don't want it to bottom out on the lath. Cut the hole a little smaller than you think you will need, then try it. If it seals, fine. If not, cut it a little larger.

We use a dripless caulk gun, but if you don't have one, keep some paper towels around to mop up any dribbles.

Insert the nozzle of the tube in a hole and press down so the nozzle seals the hole tightly. Give it one full trigger pull's worth of glue, but 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. It may drip a little out of the hole, which is where your paper towel comes in handy. Glue each hole the same way, avoiding the bad holes you circled with a pencil mark.

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 Loose Plaster to the Lath



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. We generally start at the outside and work toward the middle. This draws the plaster in gradually, reducing the risk of additional cracking.

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 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 comes 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 creates 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 a dust mask.

Step 8

Fill the Holes



The plaster is now firmly reattached 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 reattached 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. If we don't, ordinary seasonal movement in the wall will cause them to reappear as hairline cracks. To bond the plaster pieces together we are going use fiberglass mesh tape and joint compound.

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 to 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.

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 abut 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 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" wide. We use it sometimes, but most of the time, we just use nylon insect screen — the stuff you put on screen doors. It works just as well at about 1/3rd the cost. Be sure to get the coarse rather than the fine screen. Before nylon and fiberglass were invented, burlap cloth was used, and it is still an option. but it's actually more expensive these days than either nylon screen or fiberglass cloth, and harder to work with.

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 forms the keys that lock the new plaster to the lath. 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, 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 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 let everything set up for about 24 hours. A fan in the room speeds up the process.

Step 10

Apply the Finish 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 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.

You are actually going to apply two skim coats, a base coat and a final or top coat.

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 results in 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 3 foot sections across the wall. Finally, skim from the bottom of the wall up to overlaps 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 your tools and wait 24 hours before applying the final coat.

Start your final coat by first scraping down any ridges in your base 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.

Step 11

The Final Step: Sand or Burnish



Once the top coat is completely dry — about 18 hours, it is time to decide how you want to finish your wall. The choices are sanding or burnishing.

Sanding


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. Sanding drywall compound is not difficult, but it is different from sanding wood. Here are the rules:

How to Sand Joint Compound Like a Pro

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


The other finish is burnishing. We like burnishing because (1) it raises less dust and, therefore, makes less of a mess and (2) in appearance it is virtually indistinguishable from true plaster.

Burnishing is a very simple technique, but it is work, so 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" at right), 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 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.
 
And 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, you have just repaired a loose plaster wall using a plaster saving technique that works every time, but that few know about. Your wall is now as good as new, but just to be sure, you should inspect it once in a while for additional cracking. Once every 50 years or so should be plenty.

Resources


Want to learn more about traditional plaster walls and how to restore them? Here are some web sites you might want to visit.
  • Repairing Historic Flat Plaster Walls and Ceilings, Preservation Brief 21, Technical Preservation Services, National Park Service. The National Park Service 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.



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