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Mauro Henrique gives Kevin O’Connor a lesson on patching cracks in plaster and drywall. After explaining to Kevin that his paint job is only as good as the prep work, the two discuss where cracks are most likely to occur before diving into a few repairs.

Steps for Repairing Plaster Walls

  1. Start the project by cleaning up the crack with the utility knife. Look for compound, paint flakes, and other debris in the crack and scrape it out.
  2. Using the putty knife, apply a thin coat of joint compound to the crack. Start at the top of the crack and drag the compound down, pushing it into the crack as you go. Remove the excess compound by pulling the blade of the knife across the crack at an angle (not perpendicular to the crack).
  3. Apply mesh tape over the fresh compound. Starting at the top, lightly press the mesh tape into the bed of compound before using the putty knife to cut it at the bottom.
  4. Immediately apply a second coat of compound over the top of the mesh tape. Starting at the top of the tape, apply the compound and remove the excess. Wait 45 minutes to an hour so it can dry.
  5. Lightly sand the repair area to remove any high spots and ridges while also feathering the edges of the repair. Use the tack cloth to remove the dust.
  6. If the repair or crack is still visible, apply a third coat and feather the edges with sandpaper once dry.

How to Fix Small Cracks in Plaster

You can handle some small cracks with a two-coat process using a flexible patching compound, and there isn’t any sanding required.

  1. Ensure that the crack is free of debris by scraping the putty knife over the crack.
  2. Apply the flexible patching compound to the crack. Start at the top and work your way down, pushing the compound into the crack.
  3. It’s important to remove the excess patching compound as it’s not sandable. Drag the blade of the putty knife across the crack at an angle (again, not perpendicular). Ensure that there aren’t any high spots or mounds left behind before allowing the compound to dry for at least two hours.
  4. Once dry, apply a second coat of the flexible patching compound in the same manner as the first coat. Again, be sure that there aren’t any high spots or ridges left behind. Allow this coat to dry before painting.
  5. Old plaster walls and ceilings are made from two layers of plaster. The brown coat is applied over wood lath, strips that are nailed to the studs and spaced about ¼-inch apart to create keyways for the brown coat to grip.
  6. The finish coat is applied after the brown coat has hardened. The integrity of a plaster surface depends on the bond between the plaster and its wood lath. When the bond breaks, the plaster cracks.

Adhesives Can Also Do the Job Well

Rory Brennan, a Vermont-based plaster restoration expert, developed Big Wally’s Plaster Magic, a two-part adhesive, to reattach plaster without plaster washers.

  1. Using Big Wally’s is a matter of drilling a series of holes through the plaster on either side of the crack, stopping when you hit the lath.
  2. Then, vacuum out the dust and squirt the conditioner, a milky liquid, into the same holes. Old plaster will suck this stuff up like a sponge.
  3. Next, inject one full caulk-gun squeeze of the thicker adhesive into each hole, then temporarily clamp the plaster to the lath with drywall screws and big plastic washers. Neither the conditioner nor glue has any odor to speak of, and both wash up with water. The glue sets in a day or two.
  4. After it sets, back out the screws, pop off the washers, and fill the holes with joint compound.
  5. A quick skim coat of joint compound finishes the job.

A kit that includes everything you need to repair loose plaster costs a lot less than how much it would cost to remove that much plaster, haul it away, then hang and tape the wall or ceiling.

FAQs

Where should I check for cracks in plaster?

One common place to check for cracks is above door jambs and windows. They are also common at joints between the drywall and plaster. Make sure to check for these types of cracks and repair them before doing any painting.

Why do cracks in plaster occur?

As the house settles, the walls start to shift slightly. This is because the materials behind the walls are constantly contracting and expanding, which can cause cracks.

Should I use joint compound or patching compound?

Joint compound is the best choice for larger cracks, and it should be used with materials to reinforce the joint, like mesh drywall tape. For small cracks, flexible patching compound is a good choice, because it will continue to move and flex with the other building materials.


Resources

Mauro shares the best techniques for patching and repairing cracks in plaster walls. His go-to material is a flexible patch compound, Sheetrock® Brand Dust Control Patch and Repair Compound, specifically used for stress cracks on the wall. Mauro also used fiberglass drywall tape 2”x 150’ (50mm x 45.72m) by Dynamic.


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