Sooner or later, almost every plaster wall and ceiling develops cracks - if not in the broader expanses, then at least where flat surfaces join one another. Wind pressure on the house, structural expansion and shrinkage, traffic vibration, and household activities all contribute toward weakened plaster. Before any redecoration can take place, the inevitable patching must always be done.
First, clean away all material that appears loose in and around the crack. If it's a fair-sized crack, use a putty knife and a beer can opener and open the crack to its deepest part, then undercut it so that it's wider underneath than on the outer surface. Little cracks can simply be brushed clean. With a spray, a sponge or a wet rag, thoroughly dampen all surfaces of the crack. If this is overlooked, moisture from the new plaster will be absorbed into the wall, leaving the patch powdery and weak. Mix patching plaster to a thick paste and pack it into the crack with a putty knife, preferably a flexible one. (For thin, hairline cracks, use a paint brush.) Press the mix into the bottom of the crack, build up slightly more than necessary, smooth off the excess, and let it dry for 2 to 4 hours. Then use medium-grit sandpaper on a flat block to smooth off the excess. If you are going to paint later, a few strokes with fine-grit sandpaper will finish it off nicely. Before your paper or paint over this patchwork, brush on one coat or more of thinned shellac as a size coat. If there is no 'glazed' look to the size coat when dry, apply a second coat.
One word of caution - when the job is finished, don't pour excess plaster down the sink, for it will solidly block the drain pipe. If you mixed the material in a china or plastic bowl, it's easy to clean out for the next batch. Each quantity mixed should be just what you can apply in 10 minutes. After that, it starts to harden and has little holding strength left.
The general procedure for patching holes where plaster has fallen from the wall is the same as for patching cracks: undercutting, cleaning, dampening and applying new plaster. Before applying the patch, however, make sure that the lath or other plaster base has not come loose from the framing members behind. If it has, nail it back into place.
In the case of wooden lath that is broken, you will have to enlarge the hole in the plaster until two adjacent studs or joists are exposed. Then remove the broken laths and replace them with short lengths of lath nailed to the studs. If the hole is more than an inch or so in diameter, apply the patching plaster in two coats. First put on a fairly thick under-coat and, before it has quite dried, score its surface with an old comb so that the next coat will bond to it. After the undercoat has dried and set, dampen the surface and apply the thin finish coat. If the area of the hole to be patched is larger than approximately one square foot, you will find difficulty in doing an adequate repair job with patching plaster alone. One way of repairing such large holes is to apply two coats of gypsum plaster as is done in ordinary plastering and then a third coat of finishing plaster. Another way, perhaps easier, is to cut a piece of plasterboard to fit the hole, nail it in place to the lath and apply a finish coat of patching plaster.
To repair a bulge, first create a hole where the bulge appears. Do this by rapping the bulge with a hammer until the loosened plaster falls out. Be sure to knock or pry away any loose plaster around the hole so as to have sound plaster at the edges of the patch.