Q: I have a 1940s/1950s-era plaster house with many cracks around the ceilings. I really don’t think I have enough space in the cracks to shove in Sheetrock tape. Can I just apply drywall joint compound and sand it after it dries? Or do I need to do something else altogether? I want to paint over it.

A: We don’t know how one shoves tape into plaster cracks. If the cracks are big enough for drywall tape, it’s time to get out the hammer and do a little demolition.

Cracks are the result of earth movement. Successful repair will ultimately depend on the amount of movement. If the cracks are stable, simply patching them may be possible. But if cracks continue to appear, the best way to get rid of them is to place a layer of drywall over the ceiling.

We’ll tell you what’s involved with simple patching. Try it first, but keep the possibility of re-rocking in your arsenal if "simple" doesn’t work this time.

Before you begin, climb a ladder and press around the cracks with your hands. If the plaster gives, all of the joint compound in the world won’t solve the problem. If the ceiling doesn’t move, you’ve got a fair chance that patching will do the job, but keep in mind that imperfections will show.

Plaster walls and ceilings, common in the 1940s and ’50s — before the dawn of drywall — were built by nailing narrow strips of wood or metal, called lath, over the wall and ceiling framing. Several coats of plaster were then applied. The first coat was put on thick and pressed through spaces left in the lath. This formed the "keys," which kept the plaster in place.

Because the cracks you describe are many but small, the plaster keys are probably intact.

To begin, score the cracks to open them so the new mud has good purchase and adhesion to the old. Take a "tear drop" paint scraper or "five in one" tool (also a scraper) and "V out" the cracks. Be prepared for a fair amount of grit to fall, so make sure to remove the furniture from the room and cover the flooring with drop cloths. Opening the cracks in this manner will remove all the loose plaster.

Next brush or vacuum all debris from the cracks. This will ensure a solid base for the patching compound. We don’t recommend joint compound for patching open cracks. Joint compound tends to be too soft and it shrinks too much. Rather, we suggest you use patching plaster. You’ll have to mix it, but you’ll be patching with a similar material that was used for the finish coat of the room.

Application is pretty simple. Tools required include a bowl in which to mix and hold the plaster, and a trowel or drywall knife to apply the mud. Also, we suggest you wet each crack with water from a spray bottle just before filling each crack. This allows the plaster patch to blend and bond with the old plaster.

After wetting, fill each crack with patching plaster using the trowel. Leave the plaster a bit high. Let the patches dry for a couple of hours, then sand to smooth. You may find a second coat is required in some areas. If so, simply repeat the above steps.

If the cracks are larger, they might require some reinforcement. Use self-adhering mesh tape and joint compound instead of patching plaster. Apply the tape and cover it with the first coat of joint compound. Let it dry and apply a second coat. Sand the patches, apply a third coat and sand again. Finally, do touch-up filling as needed.

All that’s left is to prime and paint and keep your fingers crossed that patching the cracks will be a long-lived solution.


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