Plaster and lath walls that are damaged over a wide area may need to be completely replaced. More often, though, the damage is one small area of loose or crumbling plaster coming off the lath, which can be patched with joint compound, a type of plaster. If you fill the space all at once, the compound will crack as it dries, so you must layer it gradually and build it out to the level of the wall surface.
Instructions
1. Knock all loose plaster from the damaged area, using the edge of your 6-inch drywall knife. Make sure the remaining plaster around the edges of the hole is completely solid and that the exposed lath in the hole is free of all plaster.
2. Use the 6-inch knife to spread joint compound onto the lath inside the hole. Keep the compound thin, at least 1/4-inch but less than 1/2-inch deep.
3. Set drywall tape over the compound in strips, tearing them to size to fit inside the hole and laying them alongside each other. The tape should cover the whole surface of the compound without coming outside the boundaries of the hole.
4. Let the compound set for eight hours. Spread a second compound layer over the tape, again keeping between 1/4 and 1/2-inch thick. Let it dry for eight hours.
5. Spread a third layer of compound directly over the second layer. Let it dry. Repeat the process if necessary, until the compound has been built out to the level of the surrounding wall.
6. Apply a thin final coat of compound over the hole, using the 10-inch drywall knife. Spread the compound across the whole area and onto a few inches of the surrounding wall. Smooth until flat.
7. Let the compound set for 24 hours. Sand it lightly to smooth it out. Repaint the wall.
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