Many older homes have interior walls made of lath and plaster. Lath are strips of wood that are 1- to 2-inches wide and spaced apart. Plaster was applied to the lath and pressed between the spaces so that the plaster oozed out behind the wood. When the plaster hardened, it locked around the wood holding the plaster to your walls. Plaster walls often develop cracks or holes and need repairs. The materials you use to repair these types of walls are patching plaster combined with plaster of Paris or other setting compounds.
Vacuum or brush the crack, hole or damaged plaster to remove all loose material and all small pieces. Use a knife to open cracks to at least 1/4-inch wide. Angle cracks, holes or damaged areas to be wider inside the wall with openings narrower on the finished side of the wall. This will allow the plaster to ooze under and behind the wall to better lock the repair to the wall surface.
Scrape down the sides of cracks or around the edges of holes with a knife or a rough drywall-sanding pad. Clean off all debris. Apply a bonding agent over all plaster that is being covered. Apply fiberglass mesh tape over the surface of the exposed plaster and join joint areas together.
Mix a small amount of patching plaster in a disposable cup. Fill holes half full and press the plaster as deep into the hole as possible. Cut fiberglass mesh and press it into the top of the plaster. Allow the plaster to harden. Plaster of Paris and patching plasters harden quickly, so don't mix a lot at once.
Fill shallow cracks and work the plaster as deep as possible. Scrape off the excess with a wide putty knife. Large hole repairs with exposed lath may require rebuilding the three coats of brown, scratch and finish plaster that was originally used on the wall.
Texture your final plaster coating using a sponge for raised textures or a knife for a smooth finish. Keep in mind that plaster-of-Paris-type plastering products do not sand well. Your finish should be created with the wet plaster on the surface of the wall. Other setting compounds can be used with plaster walls if you want to be able to sand the wall after the repair.