How to Repair Plaster Walls
Stabilize sagging or cracked plaster against wood lath using plaster washers, reinforce joints with fiberglass mesh, and finish with smooth compound coats.
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🧰 Required Tools & Equipment
- ✓ Cordless drill with Phillips bit
- ✓ 6-inch taping knife
- ✓ 10-inch or 12-inch taping knife
- ✓ Triangular carbide scraper or utility knife
- ✓ Mud pan
- ✓ Sanding sponge (220 grit)
📦 Materials & Supplies
- ✓ Perforated plaster repair washers
- ✓ 1-1/4 inch drywall screws
- ✓ Self-adhesive fiberglass mesh tape
- ✓ Setting-type joint compound
- ✓ Ready-mix topping compound
⚠️ Safety Checkpoints & Warnings
- • Wear an N95 respirator and safety glasses; old plaster dust can contain silica or old lead paint residues.
- • Verify screw locations with light pressure to avoid drilling into hidden plumbing or electrical conduits behind lath.
📋 Step-by-Step Guide (6 Steps)
Open and Clean the Crack
Carve out the crack using the point of a triangular scraper or utility knife to create a wide V-groove. Remove all crumbly, loose plaster crumbs until you reach solid, sound base plaster or wood lath. Vacuum or brush out all dust from the channel.
- • A distinct V-shaped trough along the original crack line with no loose crumbs
- • Clean, sound plaster edges exposed along the entire fracture
- • Applying compound over a hairline crack without opening it up, leading to immediate refacturing.
Secure Loose Plaster with Washers
Locate areas where the plaster has pulled away from the wood lath by pressing gently around the crack. Drive 1-1/4 inch screws with plaster washers through the plaster into the lath, spacing them every 4 to 6 inches along loose areas. Tighten until the washer sits slightly depressed below the surface without crushing the plaster completely.
- • Plaster washers sunken roughly 1/16-inch below the wall surface
- • Plaster around the crack no longer flexes or gives when pressed by hand
- • Over-tightening screws until the washer snaps the plaster layer entirely.
Apply Fiberglass Mesh Tape
Center self-adhesive fiberglass mesh tape directly over the grooved crack and across all installed plaster washers. Press the tape firmly against the wall with your fingers to ensure continuous adhesion with no bubbles or wrinkles.
- • Mesh tape flat against the wall, perfectly centered over the grooved crack
- • Washers completely covered beneath the tape without tape bunching
- • Overlapping layers of mesh tape, creating thick humps that are difficult to conceal.
Apply First Base Coat of Setting Compound
Mix setting-type joint compound in your mud pan to a creamy peanut butter consistency. Using the 6-inch taping knife at a 45-degree angle, press the mud firmly through the mesh into the V-groove and washer perforations. Scrape off excess compound flush with the wall, leaving the tape embedded but barely visible.
- • Mesh grid and washer holes completely filled with solid compound
- • No thick edges protruding past the plane of the wall
- • Leaving heavy ridges of setting compound that dry hard and resist sanding.
Apply Feathered Finish Coat
After the base coat hardens, switch to ready-mix compound and your 10-inch or 12-inch taping knife. Apply a thin layer extending 4 to 6 inches beyond each side of the repair, applying more pressure to the outside edge of the blade to taper the mud to zero at the perimeter.
- • A wide, smooth compound band spanning 8 to 12 inches across the repair
- • Edges that vanish invisibly into the surrounding original wall surface
- • Keeping the knife at too steep an angle, gouging the soft center of the patch.
Sand Flush and Inspect
Once completely dry, lightly sand the perimeter and face of the patch with a 220-grit sanding sponge using gentle circular motions. Wipe away residual dust with a dry microfiber cloth and run your bare palm across the transition to confirm it feels flat.
- • Uniform matte surface with completely invisible transition lines
- • No mesh tape pattern breaking through the sanded topcoat
- • Over-sanding the middle section and exposing the fiberglass tape.
Credit: inspired by an original tutorial by This Old House — watch the original on YouTube.
Don’t have everything?
- No Setting-type joint compound? All-purpose ready-mix joint compound (allow 24h dry time between coats)
- No Triangular carbide scraper? Stiff putty knife or utility knife