Use drywall screws and plaster washers to secure plaster that has detached from the lathes behind it. There are gaps between the lathe boards, so be sure your screws hit a lathe to prevent them from popping out. Apply a skim coat of joint compound over the washer to hide it.
Repair a hole in a plaster wall using plaster of Paris mixed with an equal portion of joint compound. The joint compound will delay the drying process. Use a rough drywall screen to clear away any loose plaster around the edges of the hole. Fill the hole with the mixture, leaving a depression for a final coat. After it is dry, apply a skim coat of plain joint compound. Once it is dry, sand and paint.
A hole with broken and missing lathes behind it requires a backing. Break away the plaster surrounding the hole until the lathe is revealed. Cut a piece of wire mesh that is two inches wider than the hole. Wrap each end around a wooden dowel that is long enough to rest on the revealed lathes and secure it with wire ties. Fit the mesh in the hole and cover with an equal mixture of plaster of Paris and joint compound, leaving a slight depression. Allow it to dry, then apply a skim coat of joint compound. Allow the skim coat to dry, then sand and paint.
Repair a section of soft plaster by removing any plaster that is no longer firm and attached to the lathe behind. Clean the edges with a rough drywall screen. Fill in the hole with an equal mixture of plaster of Paris and joint compound, leaving a slight depression. Once dry, apply a skim coat of joint compound. Allow it to dry, then sand and paint.
Repair small holes in plaster with joint compound. Fill in the hole leaving a depression for a final coat. Once it dries, apply a skim coat of joint compound. Allow to dry, then sand and paint.