When plaster is exposed to lots of water from a roof or plumbing leak the brown, scratch and white coats will usually crumble and fall off the lath. This missing plaster can be replaced to restore it to its original condition, the process on this page can be used to make the repair.
Remove all the loose, crumbling plaster material using a stiff putty knife to dig out until you reach the solid surface surrounding the damage. If the damage is due to water exposure, seal the surrounding plaster with a primer-sealer to block stains and insure a good bond with the new patching material.
We use setting-type joint compound for this repair, ready-mixed is not be hard enough and it will shrink when applied in a thick coat. If you have a small repair to do use a 30 minute setting-type joint compound like Durabond45®, for larger areas use a slower acting compound, like Durabond90® to allow time to work before it sets. Check this link for handling joint compound and pour some powder into a mud pan, add water slowly while stirring with a putty knife to get a thick mud that holds its shape. Only mix as much mud as you can use right away and work quickly to apply it to the damaged area before it begins to set.
Use a small, 6 inch joint knife to apply the mud to the damaged area working from the outer edges, into the center. Push hard with the blade to make good contact with the surrounding plaster and force it into the lath. Use enough mud to fill the void to overflowing, make multiple applications if necessary to fill the whole area.
Immediately smooth out the overflowing mud to level it with the surrounding surface. Use a straight edge like a piece of 1x2 board, long enough to span the whole area. Rest the board on the surrounding surface and draw it across the wet mud to skim off the excess. This can be done in stages if necessary on large areas, let the mud set and finish filling in to build up the patch.
As mud collects on the board, stop to remove the buildup as you go. Work across the patch a couple of times to leave it level with, or slightly below the surrounding surface. If the patching compound sets higher than the level of the surrounding wall or ceiling it will result in a very noticeable bulge in the finished repair. Around the perimeter of the patch, skim the mud as thin as possible with the joint knife to help blend it into the surrounding surface.
Let the mud set for about an hour and scrape off any protrusions using the joint knife in an upward stroke. Don't try to smooth out the surface here, just knock off any peaks, ridges or other roughness in the set mud.
When the edges around the repair turn white, lightly sand there to smooth and feather it into the surrounding surface, Brush off the dust and wipe gently with a damp rag to remove the residue.
With the bulk of the damage filled, skim coat the patch with several coats of setting joint compound to build up a smooth, level finish. Mix a small batch of compound and spread a thick coat over the repair patch and well onto the surrounding wall, overlapping the edges of the filler coat.
Skim the excess mud off using the joint knife held at about 30°. Make parallel strokes across the area and return the excess mud to the pan between each stroke. Ignore any ridges left by the knife edge, you can remove them in the next step. Let the mud set for 30 minutes or until it is hard.
Scrape off any ridges or other roughness in the surface using the joint knife and then skim coat the repair area again. Apply the mud stroking in the opposite direction from the previous coat, if you used horizontal strokes last time, use vertical strokes with this application. Reverse skimming like this, with each subsequent coat, will help create a level surface and eliminate any ripples or other irregularity.
Skim coat the area until the repair is smooth, depending on the degree of damage the whole repair could require 5 or more applications of mud to build up the surface. Overlap the edges of each skim coat to make feathering easier in the final sanding.
To make sanding easier, the last skim coat can be done with ready-mixed joint compound. Allow the final coat to set and dry completely before trying to sand it.
Sand the dry joint compound lightly with 120 sandpaper. Start on one side of the patch and sand in wide arching strokes to gradually smooth out imperfections. Be careful around the edges, especially when dealing with drywall, the sandpaper can dig in while you're trying to smooth out rough edges. Lightly sand the edge to feather a smooth transition between the two surfaces.
Brush all dust from the surface and wipe with a damp rag before priming and painting. If you are using flat latex finish paint, you can use it to prime the repair. If you are using semi-gloss or other shiny paint, prime with flat latex or a latex primer first.