Plaster walls once were common in houses, but now have largely been replaced by wallboard. Plaster is a mixture of cement, sand and lime. Wallboard is made with gypsum, molded into a firm board; its joints are sealed with joint compound, commonly called "mud." Plaster is spread over a framework of lath. It forms a hard, durable surface and can be made smooth or finished with some texture. Mud is made with gypsum or some similar light ingredient, but resembles plaster, is often used to repair plaster walls and may be called "patching plaster."
Instructions
1. Use premixed, all-purpose or taping compound only for very small plaster repairs, such as holes left by removal of nails or picture hangers. Use a setting-type compound, which comes in a powder to be mixed with water, for larger repairs; it bonds better, shrinks less, dries harder and most resembles plaster. Fill small holes with a putty knife; use a drywall knife or some mason's trowel for larger areas. Mix powder compound carefully according to package directions.
2. Clean the area to be patched to remove any loose plaster or dust. Use a nail or screwdriver to slightly enlarge small holes to provide enough area for good bonding. Scrape long cracks with the edge of a putty knife or other tool to enlarge them, preferably wider on the inside of the crack. Use fiberglass mesh joint tape on wider cracks; spread a layer of compound into the crack to fill it, lay mesh tape over it and secure it with a layer of compound on top.
3. Do large areas in steps; plaster is installed in three stages, a base or scratch coat; a brown coat with sand; and a finish coat. Replicate this with compound on areas that extend down to the underlying lath or cover large wall spaces. Put down a base coat of compound, roughen the surface and let it dry. Add a second coat to almost the surface of the wall and that let it dry. Finish with a thinner final coat, feathered over the edge of the repair to blend with the plaster. Texture the final coat to match the surrounding plaster.
4. Practice texturing on a piece of drywall or other material to learn to match the original plaster for large repairs. Use a rectangular mason's trowel to skim-coat or apply a thin layer of finish mud over a broad area; this also can be used to make a variety of textures, such as swirls or "popcorn," in which mud is lifted in small peaks and then smoothed. Be prepared to scrape off compound and try again if the first try at texturing does not match.
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