DIY Repair Lath Plaster
Also see:The process on this page is for replacing crumbling brown coat, scratch coat and white coat plaster damaged by water and other causes. Use these links to repair surface damage to plaster and repair water damaged drywall.
Use setting–type joint compound for this repair, ready–mixed compound will not be hard enough and will shrink too much. If you have a small repair to do use a 30 minute setting–type joint compound like Durabond45®. For larger areas use a slower acting compound, like Durabond90®.
Repairing Deep Plaster Damage
Regardless of the cause of plaster damage, the process for replacing missing plaster coats is the same. First remove all loose, crumbling wall material and if the crumbling is due to water damage, seal the surrounding plaster with a primer–sealer to block stains and insure a good bond with the new plaster patch. Finally, fill in the missing plaster using setting joint compound to build up a smooth surface.
If the damaged area is limited to a couple square feet or so, use a mud pan to mix the mud using these techniques. If you have a very large area to fill, you can mix larger amounts of setting compound in a 5 gallon bucket.
Add water to the powder slowly and use a large paint paddle or similar stick to blend the two into a thick mud. There are also mixing attachments available for a hand drill that will make mixing large amounts of mud easier. Only mix as much mud as you can use right away and work quickly to apply it to the damaged area before it sets.
Floating Joint Compound
Apply the mud to the damaged area working from the outer edges, into the center. Fill the hole to overflowing.
Immediately smooth it out to level it with the surrounding wall. Use a straight edge like a piece of door stop molding or, on large spaces, a 1x3 board, long enough to span the whole repair. Rest the board on the surrounding wall and draw it across the wet mud to skim off the excess.
Stop to remove the buildup from the board as you go. Work across the patch, leaving the mud flush with, or just below the surrounding surface. Go over the wet mud again with the straight edge if needed to smooth out any marks from the board.
Around the perimeter of the patch, skim the mud as thin as possible with the joint knife to help blend it into the surrounding surface.
Let the mud set for about an hour and scrape off any protrusions using the joint knife in an upward stroke. Don't try to 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 dust and wipe with a damp rag.
Smooth Out the Plaster Patch
With the bulk of the damage filled, skim coat the patch with several coats of setting joint compound to build up a smooth finish.
Mix a small batch of repair compound and spread it over the surface of the repair and onto the wall, overlapping the edge of the first coat.
Skim the excess mud off using the joint knife held at about 30°, making parallel strokes across the area. Return the excess mud to the pan between strokes. Ignore any ridges left by the knife edge. Let the mud set for 30 minutes and scrape them off.
Apply the Finish Coats
Skim coat the repair area again. Apply the mud stroking in the opposite direction of the previous coat. Reverse skimming direction like this, with each subsequent application, will help eliminate any ripples or bubbles in the surface.
Skim coat the area until the surface of the repair is smooth. Overlap the edges of each coat to make feathering easier in the final sanding. The last couple of skim coats can be done with ready–mixed joint compound to make final sanding easier.
Depending on the degree of damage, the whole repair could require 5 or more applications of mud. Allow the final coat of mud to set and dry for an hour or more.
Sanding
Sand the dry joint compound lightly with 120 sandpaper. Tear a 8x11 inch sheet in half on the long edge. Fold one of the pieces in half and hold it with the fold away from you. Fanned 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 on wall repair patches, especially when dealing with drywall. The sandpaper can dig into the surrounding surface while you're trying to smooth out rough edges. Lightly sand the edge to feather a smooth transition between the two surfaces.
To sand in corners, fold the paper in half again. Open it back up to form a 90° fold and use it to sand both sides of the corner at the same time.
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.
