If you see white discolouration or what looks like a powdery, crystalline, white growth on your plaster walls, you have a moisture problem within the walls. This is , caused by soluble masonry salts within the walls leaching to the surface. Before repairing efflorescent plaster, you must address the moisture -- usually by caulking or repairing a roof leak. Once you've solved the moisture problem, fixing efflorescence on plaster is a straightforward task.
Remove all the crumbled efflorescent plaster with a wire brush. If it has pushed the paint away from the surface, remove the loose paint, then scrub away the powdery deposit.
Wash the plaster with warm water until the residual efflorescence is removed. Use a soft scrub brush if the surface is very pitted and irregular. An alkaline wash for plaster and masonry, found at paint or hardware shops, can be helpful, but it's not necessary.
Apply a coat of plaster patch or joint compound over the damaged plaster using a 15 or 20 cm (6 or 8 inch) plasterboard knife. For wide, shallow depressions left behind after you've removed crumbling, efflorescent plaster, use the widest plasterboard knife possible. They are available up to 30 cm (12 inches) wide.
Embed fibreglass mesh drywall tape along the length of any cracks while the plaster is still wet. Allow it to dry for at least eight hours.
Coat the patch with at least one more application of joint compound or plaster, extending the compound onto the surrounding surface so you can sand it flush when it dries.
Sand the repair smooth with a sanding sponge or plasterboard sanding pad. Run your hand over the repair to feel for irregularities; it's easier to feel than to see them.
Prime the repair. If there are brown water stains or discolourations, use tinted shellac or a stain-blocking primer to prevent the stains from bleeding through the paint.
Patching plaster or plaster of Paris is harder and more durable than drywall joint compound, but is harder to sand. Don't sand the patching compound between coats. If there are ridges or lumps, scrape them off with the plasterboard knife. Sanding between coats usually results in a wavy, uneven surface.
Wear eye protection, especially when scraping and sanding, or scraping above your head.