If you see white discoloration or what looks like a powdery, crystalline, white growth on your plaster walls, you have a moisture problem within the walls. This is , and it's 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.
Things You'll Need
- Wire or scrub brush
- Drywall knife
- Joint compound or plaster patch
- Fiberglass tape
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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 stores, can be helpful, but it's not necessary.
Apply a coat of plaster patch or joint compound over the damaged plaster using a 6- or 8-inch drywall knife. For wide, shallow depressions left behind after you've removed crumbling, efflorescent plaster, use the widest drywall knife possible. They are available up to 12 inches wide.
Embed fiberglass 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 drywall 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 discolorations, use tinted shellac or a stain-blocking primer to prevent the stains from bleeding through the paint.