You just gotta love plaster. That rock-hard substance, which was applied to the walls and ceilings of nearly every house in this country until the 1950s, gives us surfaces that are seamless, mold resistant, fire resistant, and noise deadening. But what to do when plaster cracks, buckles, and pops loose? It's a perplexing question for many of our readers, including Tim Thorp, whose house in Providence, Rhode Island, is filled with badly blemished plaster.
"How do I patch 100 years of gouges, cracks, and screw holes so the walls look flat and clean when painted?" he asks us in an e-mail. Here, Tom Silva shows how to repair plaster walls to make them look as good as new.
Plaster Crack Repair: An Overview
The key to any fix is to reunite the plaster with the strips of wood lath underneath. Otherwise the cracks come back, no matter how many times you patch over them. That's why This Old House general contractor Tom Silva usually reattaches lath with screws and metal washers before attempting a repair.
Recently, though, he tried Big Wally's Plaster Magic, a homeowner-friendly adhesive that uses glue instead of screws. While it costs more than the screw-and-washer method—a six-tube kit runs $120, versus $20 for 120 metal washers—the final finishing is easier and looks better because there aren't any washers to cover. Plus, a glued bond lasts longer than a screwed connection.
How to Repair Plaster Walls
1. Drill Into the Plaster
- Using a 3/16-inch masonry bit, drill a hole in the plaster about 2 inches from the crack. When you hit lath, stop—the bit won't go through wood—pull out the bit, and drill another hole about 3 inches from the first and about 2 inches from the crack. Try to hit a strip of lath with every hole you drill. If you miss, the bit will sink in right to the chuck.
- Mark such holes with a pencil as a reminder not to inject them with primer or adhesive in the next steps; try drilling again about half an inch up or down.
- Continue until there is a series of holes about 4 inches apart on both sides of the crack. Vacuum the plaster crumbs out of all the holes.
2. Prime and seal
- Put on safety goggles and disposable gloves, then spray-pump a stream of the acrylic conditioner into each of the holes (but not into any you've marked). One or two squeezes should be enough.
- Spray the edges of the crack, too, and clean up drips with a wet sponge. Wait 10 minutes for the milk-thin conditioner to soak into the plaster and wood.
3. Inject the adhesive
- Place the adhesive tube's nozzle in one of the primed holes. Gently squeeze the caulking-gun trigger until the creamy glue fills the hole and a little backs out around the nozzle.
- Do the same for all unmarked holes. Scrape off the excess and wipe the wall clean with a wet sponge.
4. Clamp the wall
- Slip a 2-inch plastic washer over a 1 5/8-inch drywall screw, and drive it into the lath through one of the adhesive-filled holes. The screw pulls the lath against the plaster's back side while the washer gives the screwhead a wide clamping surface.
- Plant washers about 8 to 12 inches apart on both sides of the crack.
5. Wipe and wait
- Wipe away any excess adhesive with a wet sponge.
- Wait a day or two for it to cure, then back out the screws and scrape off the washers. (Save them for another plaster-repair project.) Also, scrape off any dried adhesive poking out of the holes.
6. Fill the crack
- Mix up a small batch of setting-type joint compound and use it to fill the crack and all the holes. Smooth the wet compound with a trowel; then, as it begins to harden, wet it and smooth it again.
- After the compound sets, sand the area lightly, then prime and paint.