DIY Repairing Water-Damaged Plaster
Before beginning repairs to walls or ceilings that have been damaged with water, locate and stop the source of the leak, or you will be repairing the same damage over and over again. Also be sure the plaster has time to dry out completely before attempting to start repairs.
About Plaster Water Damage
When plaster gets wet the surface will usually erupt as water activates the lime in the putty coat, causing alkali salts to bubble up on the surface, and the paint to flack off as it dries. The damage may be minor, isolated to the surface layer of the wall, or it may have penetrated the whole plaster structure, causing crumbling brown and scratch coats that fall off the lath. In some cases the lath may also be damaged, with rotted wood or rusted metal under the plaster coats.
Remove the Damaged Plaster
The first step to making repairs is to remove all the damaged plaster, scraping back to the sound, undamaged coats. This includes all bubbling and peeling paint, plaster coats, and any lath involved. Use a putty knife to lift off the flaking surface. If the damage goes deeper, apply firm pressure with the blade to dig out all the soft, crumbling material. Go around the edges of the damaged area, digging with the knife until you reach hard, undamaged plaster.
With all the soft plaster removed, test the paint coat immediately surrounding the area to see if it will peel off easily. Work out from the center with a putty knife to remove all the loose paint that will come, leaving only sound finish.
Remove the Dust and Prime
Go over the damaged area with a stiff nylon brush and shopvac to remove any residual, loose particles. Wash the surrounding surfaces using a sponge and bucket of clean water to remove any residual salt deposits and let the wall dry completely before proceeding.
With the surface clean and dry, coat the whole damaged area with oil or shellac-based primer-sealer. A primer is essential to block any residual contamination that may remain and inhibit bonding of the patching compound. It will also prevent stains bleeding through the finish paint which could result in bubbling of the new paint.
Stir the primer, or shake the can vigorously to be sure the coating is mixed well. Coat all affected surfaces including the remaining plaster coats and surrounding wall paint. Let the primer dry and do a second coat. Let the second coat dry completely before beginning repairs. Shellac and quick-dry primers will dry in about an hour, while oil primer can take as long as 12 hours to dry completely.
Repairing the Plaster
Use setting-type joint compound to patch wall plaster that has been wet, ready-mixed will not be hard enough and is vulnerable to any residual moisture in the wall material. To rebuild plaster that has eroded deep down to the sand coats or lath, use the process at this link. To repair surface damage to the paint and putty coats, use the process below.
Repair this shallow damage using several thin coats of setting joint compound to fill in the missing plaster, and level out the wall. Mix a small amount of compound and cover the whole damaged area and surrounding wall with about ¼ inch coat. Use a flexible joint knife to "butter" the area like a piece of toast.
Immediately go back and skim off the excess mud, floating across the deeper parts in the middle and leaving a thin coat on the surrounding wall. Stroke in parallel rows across the damaged area, from one side to the other without stopping. Try to fill the deepest damage but don't let it buildup higher than the original wall surface. If necessary, use a long, thin board like lattice molding to scrape off the high points. Ignore any ridges left by the knife blade or other inconsistencies, when the mud has set you can scrape them off before applying another coat.
Scrape and Sand
Let the mud set until it's hard. Scrape off any protruding ridges from the first coat using an upward stroke with the joint knife. Lightly sand the area with course to medium sandpaper to remove any other bumps, etc. in the surface. A completely smooth surface is not necessary here, just remove any high points. Brush off the dust and wipe the area with a clean, damp rag or sponge to remove the scraping and sanding dust before coating the area again with mud.
Apply a Second Coat
Apply a second coat of mud, buttering the surface as before. Concentrate on filling in the deepest parts of the damage and use a lighter coat around the edges. Immediately skim off the excess mud stroking perpendicular to the first coat. If you used horizontal strokes the first time, use vertical strokes this time. Ignore any ridges left by the knife blade but keep the mud as thin as possible on the surrounding wall surface to minimize sanding when you're done.
When the second coat has set, scrape, sand and wipe off the dust as before. Apply as many additional coat as needed to fill in all the damaged plaster. As you work, don't let the mud on the surrounding wall build up to help in blending the finishes together. Don't let high points develop in the overall surface to keep it consistent with the surrounding level. Reverse direction with each new coat to help in keeping the repair level.
Sand and Paint
Let the last coat set and dry completely, and then sand with 120 sandpaper. Wipe off the dust with a damp rag and prime the new patch and surrounding wall with primer-sealer. Let it dry completely before touching up the paint.
Fixing Water Stains
Water stains on walls and ceiling caused by occasional or minor water leaks are easier to repair. Use shellac or primer-sealer to block the stain so that it won't bleed through the touch up paint. We use shellac here but a spray can of white primer will make covering with white finish paint easier.
Coat the stain twice to be sure it is completely sealed. If you're using shellac, wait 10 minutes between coats and let the second coat dry for 15 or 20 minutes. If you're using a white primer, check the can for drying time. Touch up using the original paint whenever possible.