How to Repair Water Damaged Plaster
Before beginning repairs to walls or ceilings that have been damaged by water, locate and stop the source of the leak or you will be repairing the same damage again. Also be sure the plaster has time to dry out completely before attempting to start repairs. Check the bottom of this page for a video of this plaster repair process.
About Plaster Water Damage
When three-coat plaster gets wet, the surface will usually show some visual signs. A brown stain may appear in ceilings or walls or with a heavy soaking, the surface may begin to bubble from the chemical reaction of the lime. As the surface dries, it may harden and the surrounding paint will flack off. Water damage effects may be minor, isolated to the surface layer of the wall, or it may penetrate the whole 3-coat structure, causing crumbling brown and scratch coats that fall off the lath. In some cases, the lath may also be damaged, showing rotted wood or rusted metal under the plaster coats.
Plaster that has gotten wet is best repaired using setting-type joint compound, or mud. Ready-mixed compound will not be hard enough and is vulnerable to any residual moisture in the wall material. Repair deep damage to the sand coats or lath, use the process at this link. Repair surface damage, use the process below.
Remove the Damaged Plaster
The first step to making repairs is to remove all the damaged material. This includes all the bubbling and peeling paint, crumbling plaster coats, and any degrading lath.
Use a putty knife to lift off flaking surfaces. If the damage goes deeper, apply firm pressure with the blade to dig out all the soft material. Go around the edges of the damaged area, digging with the knife until you reach hard, undamaged plaster.
Test the paint coat immediately surrounding the area to see if it will peel off easily. Work out from the center of the damage with a putty knife to remove any loose paint.
Seal the Water Damage
Go over the surface with a stiff nylon brush and shopvac to remove any loose particles. Wash the surrounding surfaces using a sponge and bucket of clean water to remove any residual lime deposits and let the wall dry completely before proceeding.
With the surface clean and dry, coat the area with oil-based or quick-dry primer-sealer. Priming is essential to block any residual contamination that may remain in the surface, inhibiting the bond with the patching compound. It will also prevent stains from bleeding through the finish paint which could result in bubbling of the new paint coat.
Stir the primer or shake the can vigorously to be sure it's completely mixed. Coat all affected surfaces and the overlap onto the surrounding wall paint. Let the primer dry completely before beginning repairs.
Quick-dry primers will dry in about an hour, oil paint may take as long as 24 hours to dry completely.
Repair the Plaster
Repair shallow surface damage using several thin coats of 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 a ¼inch coat.
Float the blade across the deeper damage to fill it in and skim harder on the surrounding walls to leave a very thin coat. Don't let mud build up higher over the repair area than it is on the surrounding surfaces.
If you have a large area to repair, use a long, thin board like a piece of lattice molding to scrape off the high points. Ignore any ridges left by the knife blade or other inconsistencies in the surface, when the mud has set you can scrape them off before applying another coat.
Let the mud set until it hardens and then use the joint knife with an upward stroke to shave off any ridges or other roughness in the surface. Use 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 it again with mud.
Apply a Second Coat of Joint Compound
Apply a second coat of mud, buttering the surface as before. Concentrate on filling in the deepest parts of the damage and keeping a lighter coat around the edges.
Immediately skim off the excess mud using parallel strokes again, but this time go in a perpendicular direction 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 build up on the surrounding surfaces to help in blending the finishes together. Preventing high points from developing in the overall surface of the repair will keep it level with the surrounding plaster. Reverse direction with each new coat of mud to help in keeping the repair level. If you used horizontal strokes first, use vertical strokes with the next coat of mud.
Sand and Touch Up Paint
Let the last coat of joint compound set and dry completely, and then sand the whole patch with 120 sandpaper. Wipe off the dust with a damp rag and prime the new patch and surrounding surfaces with primer-sealer.
Using primer/sealer instead of latex primer or paint is essential here to prevent any residual water contamination from inhibiting the new paint bond. Let the primer dry completely before touching up or applying any new wall paint.