The need for plaster repair near a window indicates a leak of some sort either around the window casing or through the wall near the window. Before you can properly fix plaster walls, you need to find and repair the leak. Once you finish that, you can repair plaster cracks with either patching compound or drywall mud. If your home has textured walls, be sure you can replicate the pattern, or your plaster repair is going to be obvious.
Scrape out the cracked area next to the window frame using a drywall trimming knife or similar sharp object. Create a V-shape along the entire length of the crack. For best results, go all the way to the lath underneath the plaster. Vacuum dust from the crack.
Examine the lath. If it appears to have weak, rotten or broken spots, enlarge the hole enough so you can remove the lath. Replace it with a new wooden or metal piece, depending on the type used in the house’s construction. You can use metal replacement lath if wood is in place, but it’s not advisable to use wood if the original lath is metal. Attach the new lath to the wall studs with nails or screws.
Dampen the sides of the crack and what you can reach of the wall under it with a clean paintbrush dipped in water. This allows the patching compound to bind to the plaster.
Push patching compound or drywall mud into the crack, using a joint knife. Push enough onto exposed lath so that it oozes out the back side, or forms a “key.” This helps stabilize the repair and bind it to the existing plaster.
Smooth off the top of the crack by pulling the flat edge of a wide putty knife along it. There may be bumps or bubbles at this point, which is normal. Allow this first coat, or scratch coat, to dry according to the package direction on the product you used.
Apply a second coat of patching compound or drywall mud over the first. This is the brown coat. Feather the edges of the repair so they blend with the rest of the plaster near the window casings. Allow the second coat to dry.
Apply a thin third, or finish, coat and let it dry. If the walls have a rough texture, mix some fine sand to the compound before applying it.
Sand the repaired area with a hand-held sander or an oscillating tool’s sanding attachment and fine-grit sandpaper. Skip this step if you added sand in the previous step.
Place painter’s tape on the window molding. Apply a coat of primer to the repaired crack. Allow the primer to dry. Use paint that matches the wall’s coloring to cover the repaired crack and allow it to dry. Apply a second coat of paint to finish the project.
Tips & Warnings
- Take care not to get plaster on the window molding as you work. If you do, wipe it off immediately with a damp cloth.
- Allow adequate ventilation when removing old plaster. Keep the windows open while sanding and working with paint products as well.
Things You'll Need:
- Drywall trimming knife
- Replacement lath
- Patching compound or drywall mud
- Joint knife
- Putty knife
- Handheld sander or oscillating tool with sanding attachment
- Fine-grit sandpaper
- Painter’s tape
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