DIY Repair Water Damaged Plaster

This page is about repairing water damaged plaster, to patch water damaged and bulging drywall use this link. Before beginning repairs to walls or ceilings, locate and stop the source of the water leak. If not stopped, the leak will certainly ruin the wall material again.

Use setting–type joint compound to patch plaster. Ready-mixed compound will not be hard enough and is vulnerable to any residual moisture in the wall material. The setting and drying times in these instructions assume the use of 30 minute setting–type joint compound like Durabond45®. When using a slower acting compound, increase these times accordingly. For help mixing and applying setting compound use the techniques on the skim coating page.

About Plaster Water Damage

When plaster gets wet it will usually erupt as water activates the lime in the putty coat, causing bubbling in the surface. This damage may be minor, surface bubbling or it can involve the whole plaster coat including the brown and scratch coats.

As the plaster dries, the putty coat may form a rock hard shell that must be chipped away with a hammer and chisel–blade joint knife. With severe damage, the sand in the brown and scratch coats may crumble and fall off the wall as the plaster dries.

How to Repair Water–Damaged Plaster

Removing Damaged Plaster

To repair plaster all loose wall material must be removed. Scrape all degraded material from the area using a putty knife. This includes bubbling or peeling paint, plaster putty coat and any brown or scratch coat involved. Remove all loose material back to the lath substrate and scrape back to solid material in the surrounding surface.

Scrape Peeling Paint

Test the paint coat immediately surrounding the area to see if it is loose. If the paint film comes off easily when a putty knife is slipped under it, it should also be removed back to sound paint.

Remove the Dust

Remove dust from the chipping and scraping process and coat the whole damaged area with shellac–based primer–sealer or an alkyd wall primer. With water damage, a sealer like this is essential to block residual moisture that would otherwise inhibit bonding of the patching compound. This could result in bubbling of the new joint compound.

Sealing the Plaster

Coat all affected surfaces including the surrounding wall paint. To help hide stains and make finish coating easier, use a primer–sealer with a white pigment added, such as Kilz® or Zinsser's B-I-N® Primer–Sealer. Let the sealer dry completely before beginning repairs. Shellac primer will dry in about an hour while alkyd primer will take about 12 hours to dry completely.

Repairing the Plaster

Use this link to repair deep damage to plaster walls and ceilings involving the brown and scratch coats. To make repairs to minor damage use the process below.

To smooth out shallow plaster damage it will take several thin coats of setting joint compound to fill and cover all the damage. Start by mixing a small amount of setting compound and spread it over the whole area using a joint knife to "butter" the surface, leaving about ¼inch of mud behind.

Immediately go back and skim off the excess mud to leave a thin coat that just begins to cover the damage. Hold the joint knife at a 30° angle and press down firmly as you pull the blade in parallel strokes, from one side of the area to the other, without stopping. Ignore any ridges left by the edge of the knife blade, when the mud has set you can scrape them off before applying a second coat.

Scrape and Sand

Let the mud set until it hardens. 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 protrusions that would cause bumps in the next coat of mud.

Brush off the dust and wipe the area with a clean, damp rag or sponge to remove the sanding dust before coating the area again with mud.

Apply a Second Coat

Apply a second coat of mud buttering the surface as before. Skim off the excess stroking perpendicular to the first coat. If the first mud coat was applied horizontally, apply this one with vertical strokes. Apply each subsequent mud coat reversing direction like this to help level any ripples and other irregularities for a smoother finished surface.

Sand and Paint

Let the last skim coat set and dry for about an hour before sanding with 120 sandpaper. Prime the new patch and surrounding paint with primer–sealer and let it dry completely before touching up the paint. Check the skim coating page for more about smoothing walls with joint compound.