Summary: Plaster walls are much thicker and more durable than plasterboard walls and are commonly found in older homes built before World War II. Because the plaster is adhered to laths, which are narrow, thin pieces of wood that run between the studs, they are more likely to require special techniques when needing repair.

Safety First

Always be sure to take standard precautionary safety steps when working with either wet or dry plaster. Make sure you have the following safety items available for your protection: safety glasses, gloves, ear plugs and a proper dust mask or respirator. Pollen masks or surgical masks are not designed for the fine gypsum dust particles that you will need to protect yourself from.

CAUTION: Environmental hazard. If you suspect that lead paint may have been used, particularly if this is an older building, or if it had been remodeled before 1970, then you should wear a respirator when working with old painted plaster. Use disposable plastic drop cloths under your work so you can just roll up the plastic and discard the lead debris in a manner appropriate to your local laws.

Before starting any plaster repair, make sure that you have prepared the wall first, so that you can make sure your new patch match the rest of the wall, as well as stick to it. This means removing residue such as loose paint, smoke, powdery surfaces, as well as cleaning the wall first, or your new plaster won’t match the stains.

Cracks

Before applying plaster, check the wall for visible cracks. To better cover the cracks with the plaster, chuck a 1/4-inch grinding wheel into your drill or power rotary tool and grind open the cracks a bit wider to increase the available surface area for the new plaster to bond to.

Loose Paint

Use a paint scraper or wire brush to remove the paint. You may have to pry off some loose paint scales with a knife blade. Wipe the wall down with a damp sponge to catch the last loose particles.

Dusty, Powdery Surfaces

Use a wire brush on the wall and then wipe it down with a damp sponge. After the area dries, check to see if the powder is gone. When you are finished, you may want to brush on a coat of fast drying oil-based primer/stain killer.

Wallpaper Bits

Make sure that all wallpaper has been removed because the little bits have a tendency to come loose or blister under your repair compound. Just dampen them with warm water and then remove them with a stiff putty knife while being careful not to gouge the plaster.

Smoke Residue

This is an oily residue condition that can be obvious or rather subtle. If you see a fireplace or a wood stove in the room; or if you know the building is heated by an oil furnace; or if the resident is a smoker, you may need to clean the walls.

If you aren’t sure about the above, then a quick test is to wipe down an area with warm water and a sponge to see if the water discolors when you squeeze out the sponge. If it does, then wipe down the whole area, changing your water as needed. It may require a couple of washings. Trisodium phosphate (TSP) is often used, but it leaves its own residue that could cause adhesion problems if it is not rinsed off the wall with clean water. There are some inexpensive commercial cleansers available that don't require clean rinsing afterward.

Note: If you notice that paint bubbles easily when you wipe old plaster down with water, you could have a problem with calcimine, which is beyond the scope of this article.

Water Stains

Brown or gray water stains may signal sagging or spongy plaster, usually in ceilings or near the top of walls. If you are lucky, you may only have discoloration and all you will have to do is brush on one or two coats of a quick drying oil-based stain blocker.

Basic Technique for Repairing Holes

Usually plaster in old homes has wood lath behind it. When plaster is missing, it not only leaves a hole, but you can often see these wood strips behind it. If they are in good shape, you can simply apply replacement plaster of a thick consistency in the gap. Many professionals use a premixed perlite-aggregate gypsum plaster that is mill-mixed with the aggregate so that simply adding water is all that is necessary before applying it.

First remove any loose plaster from around the hole, scrape the edges of the hole with a utility knife and score the edges and then dampen the hole with a plant mister before filling the hole with plaster.

After filling a hole with the first coat, scratch a crisscross pattern into the plaster to help the next coat adhere. After the plaster completely hardens, mist the patch and apply a second coat of plaster. After the second coat dries, apply a third coat and then sand and finish the wall.

Small Holes

If the hole is smaller than 2 inches, you can fill it with 2 or 3 successive layers of plaster and follow the Basic Technique for details.

If the laths are missing, you will need to put in a new backing for the new plaster to adhere to. If the hole is small enough, crumpled newspaper or steel wool are good backings that can be pressed into the hole.

Medium Holes

If the hole is about the size of a golf ball or larger, it can be filled in as directed in the Basic Technique if the laths are intact.

If the laths are damaged or missing, however, you need to put in a new backing for the new plaster to adhere to. Damaged lath should be removed as well as any loose plaster. Use a utility knife to score the edges of the hole.

Cut a piece of wire mesh slightly larger than the hole and attach a piece of wire to the center of it. Fold the mesh and insert it into the hole. Unfold the mesh and pull on the wire so that the mesh is pressed flush against the inside of the hole. You can wind the wire around a pencil, a dowel, or a scrap stick, and twist it tight so that it presses the mesh flush against the back of the hole.

Follow the Basic Technique with this one exception: after the first coat of plaster completely hardens, snip the wire and remove the brace.

If the laths are damaged and the studs are exposed, you can attach a piece of drywall to the studs for a backing and apply the plaster to the drywall the same way as to the mesh above.

Large Holes

Remove the old plaster and staple some wire mesh over the existing laths to create a backing. The consistency of the plaster mix is also very important for large holes; it should be as stiff as possible so that it will stay in the hole. Trowel the patching plaster out around the edges of the hole for a good bond.

An alternative is to patch the hole with a piece of drywall, but there could be a problem getting the drywall flush with the existing plaster. You may need to place shims behind the drywall where it attaches to the studs; or use the thinner 1/4 inch drywall and use the new plaster to build up the surface. Tape the seams with drywall tape around the replacement drywall and feather drywall joint compound as best you can to match the drywall to the plaster.

If a large part of the wall is damaged, you might want to consider covering the entire plaster wall with new drywall, or even removing the plaster wall completely and replacing it with drywall. That will be less work than trying to restore an entire plaster wall.

Once you have done it a few times, you will discover that repairing a plaster wall is a lot easier than you once thought it would be.

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