Plaster is usually installed over lath (narrow strips of metal or wood that serve as a backing for plaster.) Interior masonry and brick walls also provide a good base for plaster, and they are easier to repair because you don't have to repair the underlying lath or worry about replacing the initial "scratch" coat of plaster. Minor repairs in plaster over masonry walls may be no more involved than using some caulking and patching plaster or spackle. You may need to skim coat the entire wall if there's extensive cracking, but apart from being messy, this is not a difficult job.
Remove all the loose plaster. If the damage merely consists of cracks, use the edge of a putty knife to scrape them out.
Tap the plaster over the masonry wall with the handle of your putty knife. Listen for a hollow sound where the plaster is coming loose from the underlying masonry wall. All the loose plaster has to be removed, even if you have to whack it with a hammer to break it loose.
Clean the underlying masonry with a wire or scrub brush wherever you've pulled plaster away. Too much loose plaster dust will prevent new patching material from adhering.
Caulk cracks 1/8 inch or narrower in both the exposed masonry and old plaster with paintable silicone caulking. Apply a generous bead of caulking, and wipe off the excess with a damp rag. Allow it to dry for a minimum of 4 hours.
Spray exposed masonry with a mist of water from a spray bottle. Dampening the underlying surface will increase the bond between the masonry and the first coat of plaster.
Apply a coat of patching plaster or joint compound over the damaged areas using a 6- to 8-inch drywall knife.
Embed strips of fibreglass drywall tape into the wet plaster over cracks larger than 1/8 inch wide. Smooth out excess with the drywall knife. Let this dry for at least 8 hours.
Apply at least two more coats of plaster or joint compound over the repairs, extending it onto the surrounding wall so you can sand it flush after it has dried.
Sand the repair with a medium/fine sanding sponge or mesh drywall sanding pad until it's smooth.
Prime the repair with latex (water-based) primer before painting. Check the repair in strong light after the primer has dried for imperfections like ridges and air bubbles. Fill, sand and reprime if necessary.
Plaster or joint compound is grey when wet, and white when it's dry.
Wear eye protection whenever you're scraping or sanding plaster.