A plastered wall could be almost indistinguishable from one covered with shingles until cracks appear. In drywall, cracks often follow the joints between drywall sheets, but in plaster, they could run in any way, and they tend to look more frequently. They occur because plaster is brittle and can not withstand movements in the framing brought on by moisture and settling. You may repair these cracks using either plaster or drywall joint compound, but they will keep coming back in the event that you don’t tape them first. Self-adhesive fiberglass net is the best tape for the job.
Rake over the damaged plaster with a paint scraper. Do not use the tool to scrape — simply draw it over the damage to eliminate loose material, which should fall out on its own.
Unroll enough self-adhesive fiberglass mesh tape to cover the crack. If the crack curvescut another piece for each leg of the curve — do not try to follow a curve by bunching up one piece of tape. Cut the tape as needed with scissors and stick it to the wall, overlapping pieces as required to cover the crack.
Cover the tape with plaster or drywall joint compound. Examine the container if you use plaster — to ascertain whether you should wet the wall prior to applying it. If the directions specify that you do need to moisten the wall, do it with a sponge soaked in plain water.
Apply 1 coat of plaster or drywall joint compound over the tape. If you use joint compound, then spread it with a 6-inch drywall knife and scrape on the surface lightly to flatten it. If you use plaster, use it with a plastering trowel, putting it off the tape and feathering the edges into the surrounding wall in addition to possible.
Apply another coat of joint compound following the very first one dries, using an 8-inch knife. Smooth it on and scrape off the excess, feathering the edges into the wall. If you are using plaster, apply a thin layer over the prior one after it has dried to fill holes and voids.
Employ one or two coats of joint compound, using a 10- or 12-inch knife. Scrape the borders of each coat carefully to feather them into the wall and create the repair invisible. If you are doing the repair with plaster, you should not have to use any more after the second coat has dried.
Sand the repair lightly with a sanding sponge when the plaster or joint compound has established. Prime the joint compound or plaster with polyvinyl acetate primer prior to painting the wall.