How to Repair Drywall Water Damage
When water leaks onto drywall it can cause minor damage like stains on the paint finish; very severe damage like bulging in the wallboard; or some degree of damage in between such as bubbling joint compound and peeling tape and paint.
How to Repair Minor Water Damage
Remove bubbling joint compound and paint using a putty knife to scrape off all the damaged material. Remove peeling joint tape by cutting it free. Trace the tape back to just outside the damaged area and cut straight across it with a drywall knife. Pull the loose tape off and scrape away any loose joint compound.
Brush or vacuum away the scraping dust and prime all the affected wallboard with primer-sealer to block stains and seal the surface. Let the primer dry completely before replacing missing tape and patching.
When the sealer is dry, repair any surface roughness and damage using this bubbling paint repair process. Re-tape any exposed drywall seams with fiberglass mesh joint tape and use these crack repair techniques to cover and finish the taped seams.
How to Repair Major Water Damage
When exposed to a lot of water, drywall tends to warp as the gypsum swells. When it dries, the gypsum may harden into a bulging wall or ceiling surface. This bulging gypsum will tend to be harder than before it got wet. This will be true of plaster with gypsum board lath also. Bulging gypsum like this will always need to be cut out and replaced to restore a flat surface.
Draw a square around the damage and cut out the affected drywall. Trace the nearest existing drywall seams and wood framing to guide your cut. Using the existing layout of the drywall will give you framing to attach the replacement piece and make it easier to cut the hole square.
Score repeatedly with a sharp blade until you cut completely through the gypsum and backing paper. Remove any screws or nails holding the drywall up and pull the damaged area free.
Scrape off all damaged wall material surrounding the new hole and prime the whole area with a primer/sealer.
Cut a new piece of drywall of the same thickness and fit it into the space. Match the edges of the patching piece to the surrounding drywall edges. For example, if the drywall edges around the square hole are open with the gypsum core visible, cut the patch with open edges to match. This will prevent an uneven joint where the two meet.
With gypsum board plaster, use a slightly thinner piece of drywall to patch it. This will place it a little below the surrounding surface allowing room to build up over the patch with joint compound and restore a matching smooth surface.
Tape the joints around the new piece of drywall with mesh tape and apply a first coat of mud using the techniques for finishing drywall seams. If the new patch is level with the surrounding surface, finish the seams as described. Let the repair dry completely and sand it smooth. Wipe away the sanding dust and seal the whole repair with primer/sealer. When the sealer dries, touch up the paint.
If the patching drywall is below the surrounding finished surface because you're dealing with gypsum board plaster, apply the tape so it follows the contour of the seam and sticks flat to each surface. Then apply a thick coat of mud to the seams and skim off the excess working across the tape to smooth it out onto both surfaces. Use light pressure to avoid pulling the tape off, but press hard enough to leave only a thin covering. Let the mud set and level out the uneven joint in the next step.
When the seams dries, apply a coat of mud to level out the patch. On large areas, use a slow acting setting compound like Durabond90® to give yourself time to work. Use a broad joint knife to apply a thick coat of mud that fills in over the drywall patch and overlaps the surrounding surface.
Immediately skim the excess mud from the patch to level it. Use a straight edge like a 1x3 long enough to span the whole repair and skim off the excess mud. Rest the board on the surrounding surface and draw it across the patch, stopping to remove the collected mud as you go.
Work across the patch to smooth out the surface. Around the perimeter, use the joint knife to blend the mud with the surrounding surface. If necessary, go over the wet mud a second time with the straight edge to smooth out any missed spots. Ignore minor voids or ridges in the mud coat, they will be filled in the next application.
Finishing Gypsum Board Plaster
Let the mud set for about an hour and scrape off any protrusions using the joint knife. Don't try to completely smooth out the surface here, just knock off any peaks or ridges in the mud. When the edges of the repair turn white, lightly sand there. Brush off the dust and wipe the surface with a damp rag to remove any joint compound residue.
Apply a thick coat of mud over the whole patch again, overlapping the edges of the previous coat. Immediately skim the excess mud off using the joint knife. Make parallel strokes across the patch using firm pressure with the knife. Ignore any ridges left by the edge of the blade, you will scrape them off when the mud sets.
Let the mud set and use the joint knife to shave off any ridges in the surface. If necessary, lightly sand to smooth around the edges and wipe off the dust. Use this process to apply as many subsequent mud coats as needed to build up a smooth surface over gypsum board plaster.
With each coat, skim in the opposite direction from the one before. If you skimmed vertically, apply the next with horizontal strokes. Reversing direction like this with each application will help eliminate any ripples or uneven surface.
Always overlap the edges of the previous coat of mud and skim it out as thin as possible around the perimeter to make final sanding easier. Allow the final coat of mud to set and dry completely before final sanding.
Sanding the Patch
Sand the dry joint compound lightly with 120 sandpaper. Tear a 8x11 inch sheet in half to form two 8x5½ inch pieces. Fold one of the pieces in half and hold it with the fold away from you. Fan out your fingers to apply even pressure and use a light touch. Start on one side of a repair and sand in wide arching strokes to gradually smooth out imperfections.
Be careful sanding around the edges of patches on drywall. The sandpaper can dig in and tear the paper surface while you're trying to smooth out rough edges. Lightly sand across the edges to feather a smooth transition between the two surfaces.
Brush all dust from the surface and wipe with a damp rag before priming and painting. If you are using flat latex finish paint, you can use it to prime the repair. If you are using semi-gloss or other shiny paint, prime with flat latex or a latex primer first.