Plaster repairs can seem daunting, especially when you've attempted to patch a crack and it reappears within a year. Here's how to make repairs that last.
Use a putty knife to apply spackling compound, available in small cans for touch-ups. Let dry. If the first coat shrinks as it dries and leaves a slight indentation, apply a second coat.
Sand lightly with fine sandpaper. Touch up with primer and paint.
Remove any small, loose chunks of plaster and patch the holes (see "Fill holes and areas where plaster is missing," opposite page). Reattach larger sections of plaster with screws and plaster washers, spaced a few inches (about 7 cm) apart, using a drill and driver. For large, loose areas, install the washers in concentric rings, starting where the plaster is firmly attached and working inward. Cover the area with joint compound.
To reinforce plaster along large cracks, install screws every 6 inches (15 cm) on both sides of the crack and about 1 inch (2.5 cm) away from it (see A). Drill pilot holes with a 1/8-inch (3-mm) masonry drill bit. Cover the area with joint compound.
Use a setting-type joint compound such as Durabond 90 for all repairs other than tiny holes. Mix the compound in a bucket with water according to the directions. Depending on the quantity, use a potato masher, paint-mixing drill attachment or paddle mixer.
Transfer a working supply of the compound into a mud pan, a drywall specialty item.
Fill a hole with compound to within 1/8 inch (3 mm) of the surface using a 6-inch (15-cm) spackle knife. Then scratch the wet compound in a crisscross pattern with the edge of your knife. When the scratched coat dries, apply a leveling coat.
For larger areas of missing plaster, measure a piece of drywall to fit the hole, cut the piece out with a utility knife, then screw it to the lath (see B). Apply the compound in two stages and extend it about 2 inches (5 cm) onto the surrounding surface. Embed paper drywall-reinforcing tape in the compound and smooth with a spackle knife or wide trowel (see C).
Apply two or three additional coats of compound, allowing complete drying between coats and feathering each coat over a wider area than the preceding one. Drying time varies according to the type of compound as well as the humidity and ventilation.
Use a pole sander or a similar pad sander with very fine (150-grit) sandpaper to smooth the compound after it has dried.
Apply a thin coat of compound on the wall along the crack.
Embed paper drywall-reinforcing tape in the compound immediately and smooth with a spackle knife.
Continue as described in steps 3 and 4 of "Fill holes and areas where plaster is missing," above.
Setting compounds dry very fast and hard. Don't mix more than you can use in the time indicated on the packaging (30, 45, 60 or 90 minutes), and don't allow the compound to dry on your tools. Don't use fiberglass mesh tape or premixed joint compound. Although they are easier to work with, repairs made with them won't last.
Wear a dust mask and, if sanding overhead or high on walls, goggles. Don't use a household vacuum for plaster dust, which is so fine that it gets past filters and into the motor.