If you’re dealing with damaged lath and plaster walls—for example gaping holes in the wall because of electrical work—here are techniques you can use to repair them. First, you’ll repair the lath and then patch the hole.
How Do You Repair a Hole in Lath and Plaster Walls?
For lath and plaster repair, it’s important to use good techniques and the right materials. It’s better to use a softer, slower-setting lime-based plaster, like the one developed for Big Wally’s line of plaster-repair products, takes about an hour to set, isn’t prone to cracking or delaminating, and needs no sanding, which keeps dust to a minimum. Don’t use so-called patching plaster sold at home centers, it’s much harder than the wall’s original plaster, and it sets in the blink of an eye.
If the lath is in good condition and you need to repair holes, patching is a matter of troweling on layers of mud. Into the 20th century, plaster was applied over wood lath spaced half an inch or so apart over the studs or ceiling joists. Several coats were applied, the first keying into the spaces between the lath to make a stable base. Finish coats came after the base had dried. It’s a common practice to replace missing plaster with a piece of blue board (a wall board made to be coated with plaster), but troweling on base coat is faster.
How to Repair Lath and Plaster in 6 Steps
1. Fix the Lath
Replace any missing lengths of lath, and refasten all loose pieces. Use drywall screws, and always drill pilot holes to avoid splitting the wood. If there’s no stud to anchor a lath end, slip a piece of lath into the cavity and screw it to the back of the existing ones, parallel with the studs. Then you can screw new or loose lath to it, as if it were a stud.
2. Stabilize the Wall’s Edge
Using a 3/16-inch masonry bit, drill holes every 3 inches around the damaged area, about 1 inch from the edge. Drill until the bit hits the lath. If it misses, pencil a mark by that hole. Vacuum the holes and spray them with plaster conditioner. Wait 15 minutes, then fill all unmarked holes with a squirt of plaster adhesive. Wipe off the excess with a damp rag.
3. Wet the Substrate
Dry lath and the exposed plaster edge will suck water out of wet plaster before it has a chance to harden. To prevent this, saturate those surfaces with conditioner. Wipe up any drips or overspray with a damp rag. Wait until the conditioner dries—about 20 minutes—before going to Step 4.
4. Apply the Scratch Coat
In a clean bucket with cool tap water, mix Plaster Magic patching plaster to the consistency of natural peanut butter. Using a margin trowel, smear the mix against the old plaster edge, as shown, and over the lath. Keep this layer recessed by half the thickness of the original plaster. Rough up the patch by scratching it with a scarifier, then scrape any blobs off the wall.
5. Trowel on the Second Coat
Wait until the scratch coat is firm to the touch—about an hour—then mix up a new batch of plaster to a slightly thinner consistency—closer to that of buttercream frosting. Use a plastering trowel to smooth the mix flush with the wall surface; scrape any excess off the wall. Wait another hour for that coat to set.
6. Smooth the Top Coat
Using a 6-inch taping knife, scrape the patch area smooth, and cover the patch with a thin layer of ready-mix joint compound. Let dry overnight. Gently scrape it smooth and apply a second coat. On the third day, spread another coat, and after it dries, smooth it gently with a damp sponge. The patch is now ready to prime and paint.
How to Patch Plaster Walls if the Lath is in Good Condition
If the lath behind the old plaster is sound, patching is a matter of troweling on layers of mud. In this video, This Old House host Kevin O’Conner shows you how to patch holes in plaster walls.
Patch Holes in Plaster in 7 Steps
- Force base coat into the lath. With all the loose old plaster removed, use a flat trowel to coat the exposed lath with plaster base coat, being sure to work the material into the spaces between the lath. The plaster that goes between the lath is called “keys”, and it locks the material in place. Use a stiff mix so that it won’t sag before it sets. Nylon fibers added in while mixing the base coat add strength.
- Fill the area so it’s flush. Once the base coat keyed to the lath, use the flat trowel to fill the area with more base coat. Once the base coat is flush with the surface of the remaining old plaster, smooth it well with the trowel to minimize the need for sanding.
- Paint on a bonding agent. When the base coat is dry, brush a plaster bonding agent over it and the surrounding old plaster. Essentially a water-based glue, the bonding agent ensures good adhesion between the old work and the new.
- Tape the joints. Go over the joint between the new base coat and the old plaster with nylon mesh tape. This step will reduce the chances of any cracking that happens in the base layers from telegraphing through to the finished surface.
- Coat the tape. Using a setting-type veneer plaster, use a taping knife to trowel a thin layer over the mesh tape and surrounding wall. Setting plaster hardens quickly, allowing for the next steps to happen in short order.
- Finish with joint compound. After the veneer plaster sets, knock down any high spots with sandpaper, being careful not to sand through to the mesh. Finish the patch with two coats of joint compound, allowing the first to dry before smoothing on the second. The idea is to build up a large enough area to allow it to be blended to the existing wall without creating a noticeable high spot.
- Sand smooth. After the second coat of joint compound has dried, blend to the old plaster using 220 grit sandpaper. Focus on blending the edges to the existing wall first, then smooth out the center of the patch.
As you could see, patching missing plaster follows a similar path as the original installation. A base coat is followed by several finish coats and a quick sanding to make old walls look new.