Taping Plaster 200. Locking Up BAD Cracks

In my experience with taping plaster cracks, the usual treatment with paper drywall tape is ordinarily sufficient to secure the cracks and prevent their re-opening.

But sometimes you come across those "cracks from hell" that just keep cracking again. What is happening, probably, is some persistent structural movement under the surface that builds up stresses strong enough to rip your tape job.

So now you are back to square one. How do you keep that crack CLOSED?

Now, obviously, if the structural movement is strong enough, no amount of tape and mud will suffice. But often, it's not quite so severe. You just need a sturdier reinforcement plan.

I have written elsewhere about using a combination of strips of fiberglass tape and paper tape on those tough cracks. That often is adequate.

But I may want to take this to a new level, as added insurance. So here is how I approach it.

First, I make sure the areas alongside the crack are clean and dust free. Then I paint on liquid plaster bonding agent on both sides of the crack out about a foot or more. This will make for a good extra tight bond between my mud and the plaster surface. Let the bonder dry a half hour or so.

Next, I make sure the crack is free of any loose material. I prefill the crack with hot mud, forcing it in so the crack is well filled. After it hardens, if it shrank any, I give it one more fill coat. Let harden.

Now, my super reinforcement steps.

One, tape the crack with short strips (3 or 4 inch) of paper drywall tape, laid across the crack in parallel fashion, not overlapping but close side by side. Use hot mud for extra strength. This provides the "understory" of your crack treatment.

Two, get some fiberglass window screen material. Cut a strip about the length of your crack (or cut off ends of the roll) and about two feet wide. Lay down a bed of hot mud wide enough to accommodate the width of your screen material. Be consistent - no skips.

Place the screen material along the crack, centered on the taped joint. Place your fingers in the center to hold the material down, then with the other hand wipe with your six or eight inch taping knife from the center towards the edges, working out the excess mud. Do this along the length of the crack, being careful not to get any wrinkles. You want the screen stuff well bedded down, but not much mud on top.

Why the window screen material? Because it is wider than fiberglass drywall tape, and much stronger. This is the key to the whole system. (But don't skip the paper tape step - this too is essential.) The heavier screen stuff provides a lot of sheer lateral strength. Once you top it with two coats minimum of hot mud, you've got one heck of a lot of holding power.

If you are really worried about a particular crack, there is one additional step you can take. And that is -- apply a second layer of window screen on top of the first (when it is hard) and before the topping coats. This may be overkill. And your treatment area is getting thicker with each layer and you will have to take extra pains to hide the hump by feathering out farther on each side of the treatment area.

So that's basically it. Is it guaranteed? No, but in most cases it will hold.And if it doesn't? Consult a good carpenter. It may be necessary to remove plaster and see what can be done to beef up the framing to stop movement.

But . . . most often, we can hope it doesn't come to that!

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