Repairing a plaster corner on a wall is slightly harder than repairing an area on a flat wall. However, the principles are the same. While repairing the corner, it is important to keep it sharp so the corner does not turn into a curved area. The best way to accomplish this is with a putty knife or other flat object held in the corner to keep the corner intact as you repair the plaster.
Scrape or chip away all damaged plaster with a putty knife or chisel. You can tell the plaster is damaged because it will easily crumble away if it is dry, or it will be soft and mushy if the plaster is wet. Eliminate the source of the moisture damage to prevent the problem from occurring again.
Apply primer sealer to the area using a paintbrush to prevent water from damaging the plaster again. Allow the primer sealer to dry for a minimum of two hours.
Mix two parts plaster powder with one part water in a bucket. Mix very small amounts, such as 1 cup of plaster and 1/2 cup water. Mix until it is the consistency of thick mud.
Spread the plaster in the hole or crack, starting from the outside and working inward. Fill in nearly the entire hole at one time. When you reach the corner, make sure to keep corners sharp by placing a putty knife in the corner to retain the angle.
Skim off any additional plaster from the corner with doorstop moulding. Feather out the edges of the plaster to blend with the original plaster. Allow the patch to dry for two hours.
Sand the surface of the plaster lightly once it turns white. Use a damp rag to wipe away dust.
Apply a layer of joint compound over the plaster. Continue to keep the corners sharp, and use a large putty knife to smooth the compound. Allow the compound to dry overnight.
Apply repair compound on top of the joint compound to give the top of the plaster the same texture as the original plaster. Use the same texturing tools. If the plaster is smooth, then use a putty knife to smooth the plaster. Allow the repaired corner to dry for 24 hours.
Sand the top of the plaster lightly with sandpaper to prepare it for painting. The final finish should be smooth unless the wall is textured.