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Plaster 101: DIY Repairs, Tools, and Techniques

See common problems with plaster in historic homes and which DIY tools and techniques are most effective for repairing

Many houses built before about 1950 retain at least some of their original plaster walls. If sections or entire walls need attention, don't be intimidated by the idea of ​​working with old plaster. In many cases, plaster problems can be solved. It is much easier to repair cracks or patches with this versatile material than with drywall.

The most common type of plaster damage is cracking, which progresses from the surface and is easily repaired to deeper, larger cracks that require multiple repair techniques. More serious repair issues include delamination, protruding walls, and key breakage, where the plaster has come loose from its grip on the lath substructure.

What is plaster?

Applied with skill and skill, interior plaster is an almost magical building material for creating and lining walls and ceilings. Historically, plaster is made of dry components including sand, lime, and cow hair (or straw or manure) mixed with water to create a moist plastic carrier that hardens and hardens as it dries.

About 150 years ago a new component, gypsum, was added to the sand and lime mixture to allow the plaster to harden faster and allow faster working times.

What is a 3-coat render

A traditional multi-coat render system starts with a batten substructure - thin wooden battens or wire mesh - which is attached to the wall frame. Three coats of wet plaster are applied in layers to the cleat: the scratch coat, the brown coat and the finish coat.

The scratch

The scratch sets in quickly.

The plasterer densely forces the wet mix through the slots in the lath to form "keys" that harden and lock the coating in place as it hardens. The top surface of this anti-scratch coating is then marked to prepare it to receive the second coating or brown coating.

The brown coating

As the brown coating progresses, the plasterer levels the surfaces of the walls and creates right and plumb angles.

The Topcoat

After the brown coat has cured, the finish or topcoat continues, smooth or with a variety of textured finishes. The result is a hand-crafted top surface that offers surprisingly good insulation and soundproofing qualities.

What is drywall?

Known as drywall, gypsum board, gypsum board (a trade name), rock, or drywall, drywall is a uniform panel of gypsum plaster pressed between sheets of thick paper. It's not as old as plaster, but drywall has a surprisingly long history, with the earliest forms dating back to the late 19th century. In the United States

, began its inevitable march towards residential walls and ceilings between 1910 and 1930.

Offered in a standard size of 4′ x 8′ and thicknesses from ¼" to 5⁄8", drywall is relatively easy to install over large areas, especially compared to plaster. It can be nailed or screwed to wooden or metal posts. Once the joints are taped and coated with joint compound, the drywall is ready to be painted or wallpapered.

Manufacturers offer drywall for different uses and environments: there is fire, such as moisture-resistant varieties or mold and rock that serve as the basis for laying the tiles .

This makes it easy to adapt drywall with specific features to areas where moisture or mold is a problem. For a bathroom where the original plaster is long gone or compromised by water damage, for example, greenboard is an excellent choice.

Problems with plaster

Repairing cracks

Cracks can be caused by sagging foundations, undersized joists, seasonal changes and moisture and poorly made repairs. The cracks can range from the size of the hairline to very wide. They can follow structural members in a straight line or run diagonally across the wall surface.

Cracked wall repair is a skill that falls somewhere between the trades of plastering and finishing drywall. In other words, a willing beginner can make minimal repairs and, with practice, progress to more difficult projects.

Hairline Cracks

If a hairline crack is a hair has been noticeable for a period of time and it is not moving, remove the loose material and fill the space with one or two coats of pipe dope or filler (or use an elastomeric product such as Krack Kote). Allow each coat to dry, then lightly sand before applying the next coat. Sand the piece smooth with fine grit sandpaper on an abrasive block.

Larger cracks

For larger cracks or previous cracks that didn't work, use the tape and float technique. Press hard on both sides of the crack first to make sure the plaster doesn't move. Then check to see if one side of the crack is more prominent than the other.

If you are facing a large network of cracks or crevices in random patterns called map cracks, cover the walls with Adhesive-backed fiberglass mesh sheets with an elastomeric coating, such as Nu-Wal, following manufacturer's instructions. There may still be ribbons and subsequent fillings once the product is in place.

Move the cracks

If the plaster moves or if one side of the crack has a visibly higher relief than the other, dig the crack out of the high or free side with a utility knife or crevice opener, small hand tool. Remove enough material to reach sound plaster.

Keep in mind that the plaster will continue to move over time. While it is simple to fill any crack with joint compound, unless the area around the crack is stable and isolated from other problem areas, the cracks will tile again.

Most professional plasterers recommend repairing the free joint or high side of the crack surface with gypsum washers, very thin perforated discs (see below) which reattach the plaster to the lath and drywall screws before filling the crack.

Start by drilling small pilot holes at least 2″ from either side of the crack. Then secure the washers with 2" screws.

Although the washers flatten out when the screw is inserted into place, they will still sit slightly above the wall surface. In most situations, it is possible to achieve an almost invisible stain by fading the repair a few inches wider than usual.

Two methods of sealing a crack

There are two methods to seal the crack once the wall is anchored.

They are:

The Inverted V Method

Digs the crack along its length in an inverted V shape and sucks out any residual debris. This should create a foot for the patch.

Provided the crack survives without crumbling, fill and secure as if it were a drywall joint.

Wet the dressing first. lath and plaster around the cleaned crack to help the patch adhere (old, dry materials tend to suck moisture out of the new plaster before it can fully harden).

Then fill the crack with a coat or two of hard-mix joint compound to lift and level the crack.

Also ensure that the new plaster adheres firmly to the batten by pressing the material through the openings in the battens. Otherwise, the repair will fail.

Fill the gap with two thinner coats of joint compound, sanding between finishes.

Finish with a final coat of joint compound or Easy Sand and touch up

The bonding method

Use a PVA plaster adhesive ( polyvinyl acetate) on the edges of the damaged plaster and on the exposed battens before sealing the crack.

The bonding agent acts as an emulsifier to bond new plaster to old without the haphazard method of wetting old lath and plaster. Then fill the hole as above. Allow each filler application to dry and sand between coats.

Carry out small repairs

For small jobs, it is not necessary to mix a traditional lime-based plaster and sand. Unless the house is very old or historically significant, a premixed joint compound like Murco Mud works well.

If there are multiple cracks or areas that need to be repaired, use a dry joint compound that can be mixed to the desired consistency, such as Durabond, available at most building supply stores and online. It comes in 20, 45 and 90 minute varieties. It hardens quickly and is difficult to sand, however, for finish work, use a premixed joint compound or related product called Easy Sand, a light-curing mud with cure times of 25 to 90 minutes.

For scratches or brown layers, choose a base filler, such as Structo-Lite, a perlite-aggregate gypsum much lighter than a traditional sand-based filler.

How damaged is your coating?

Faulty plaster creates a huge mess, which is why so many people throw up their hands and call a drywall contractor. If your goal is to save as much of the original plaster as possible, first identify the depth of the damage.

Peeling: Failing of the finish coat

Often only the top layer of plaster fails. This is called delamination. If so, scrape off the loose top coat with a putty knife and brush off the loose material.

Leave any healthy material intact.

Apply an adhesive plaster (Plaster-Weld is a registered trademark) to the damaged areas. Carefully follow the adhesive's instructions regarding wall preparation and curing times. Then apply a new coat of finish using veneer plaster or multiple coats of gypsum compound.

Veneer plaster creates a thicker build in one coat than drywall mud.

For Best Results:

Mix small amounts until the consistency of mashed potatoes.

Bring the plaster directly to the wall with the trowel and apply it in an arcing motion. (This may take some practice.)

When the plaster begins to dry, brush it with a layer of water and smooth it out quickly. This will help the plaster harden to a smooth finish.

Start with the anti-scratch coating

Once you have removed all the debris and repaired the slat system, begin the process of plaster with the anti-scratch coating:

Use the plaster washers to secure the edges of loose or suspect plaster to the frame or batten below.

Screw the washers into the plaster about 2 inches apart, placing them about 4″ apart. Use all necessary washers until the cast stops moving. On ceilings, you may want to anchor the entire ceiling with plaster washers, placing them along the joists about 1′ apart. Secure with 2″ drywall screws.

Once the washers are in place, prepare the area to be repaired by coating the edges and cleat with a bonding agent.

When dry, mix one batch of Structo-Lite, a lightweight perlite-gypsum coating product. Blend until the consistency of mashed potatoes and only as much as needed for the patch. Structo-Lite should not swell, run or fall off the trowel.

Apply the plaster with an arcing motion taking care to press the material through the wood or metal lath (see below).

Aim for a layer no thicker than the depth of the existing anti-scratch coating (about 1⁄8″ to ¼").

When it begins to solidify, score it with a nail to prepare the surface for the next coat.

Allow the anti-scratch coating to cure for 24 to 48 hours.

Step 2: the brown coat

Use the same coating to the brown coat and apply it more or less the same way:

Prepare the surface by wetting it thoroughly with water before you start.

Make the hand as horizontal as possible to provide a solid base for the finish.

To achieve this, run a putty knife over the entire patch once it's in place.

Keep the brown layer at least 1⁄8″ below grade when it begins to settle.

Allow 24-48 hours to cure before applying top coat.

Step 3: the top coat

For the top coat, use Durabond or coated coating:

Moisten again the surface with water. This will help the new coating adhere without drawing too much moisture from the first and second coats, which can cause cracking or breaking.

Finish plaster should harden as it is leveled and sanded.

Add small drops of plaster to fill the cavities, then smooth the edges.

To create a smooth finish, spray a light mist of water on the surface and make a few final passes with the trowel.

Straighten edges or corners with an edge trowel.

li> Allow the topcoat to cure for at least 48 hours (some coatings will take longer).

Next check for shrinkage, where the new layer has shrunk from the old one.

Tape cracks with fiberglass mesh and joint compound as for any small crack in plaster.

How to Use Skim Coating

Even when plaster walls are in good condition, often you can see signs of repeated repairs, nicks or surface roughness. You can refresh them with the foam filler.

Foam plaster is a plastering technique that consists of applying a very thin layer of plaster (1⁄8″ to 3⁄16″) on a surface.

While recipes and preferences vary, a skim coat should be stiff enough to keep it from slipping off the trowel when turned over.

Assuming you are plastering freshly repaired plaster walls that have not been wallpapered or painted, start by applying an adhesive agent. While this sets the stage for the new coat, it also makes the surface a little slippery and also means you'll have to work fast.

I recommend the following technique:

Take a dollop of plaster mix on a trowel and spread it on the wall using one smooth motion of arc.

Move quickly to cover most of the area allowed by the amount on the trowel.

Then use the corner of the trowel to scrape off any plaster that does not initially adhere to the surface.


Use a crimping knife to remove buildup from the trowel, then scrape the trowel again with the hard corner.

Once you've completed your steps in one area and scratched them, move on to another.

Try to keep a wet edge on the trowel like you would a paintbrush. Help wet the edges of your tools occasionally by drying them in a bucket of clean water to remove any buildup. Also, as the mixture tends to harden quickly, scrape it into the bucket and get out a new, fairly thick measuring cup.

Openings and corners

At openings such as boxes joint, spread the compound on the quick use of the joint knife. Clean from the center of the opening, applying more than necessary.

Use the trowel to scrape straight through the opening and remove as much material as possible.

For corners, use the joint knife to spread the compound into the corner, then peel it off. Once you fill in the corner from top to bottom, there will be ridges and lines.

Once the plaster has hardened, follow the same procedure as for a plaster plaster: carefully check the walls for gaps and high spots, circle them with a pencil and use a knife to quickly tape repairs. As always, sand the top coat smooth and touch up as needed.

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