A straight crack that appears on your drywalled wall, then reappears with time whenever you plaster or paint over it, is going to need a more permanent fix. Unlike old-fashioned plaster walls, which can crack anywhere, drywall will only crack at the seams (where joint compound is holding them together). It means the tape used between the sheets has split in half. Resetting a new piece of tape in a new layer of joint compound will solve the problem. It's a dusty, messy job that will take a few days.
Instructions
1. Take off the top layer of paint and dried compound (plaster) at the cracked area using your vibrating sander. Run the sander lengthwise over the crack, and for a foot or two on either end of it. The sanding should expose either paper or mesh drywall tape that's been split at the seam.
2. Pull the tape off, using your razor knife to cut them at the ends so it doesn't continue to pull off compound throughout the wall.
3. Press a strip of mesh tape lengthwise over the crack, covering the whole sanded area in the process.
4. Spread a narrow strip of joint compound over the area with your drywall knife, covering the crack and the sanded area on ether end of the crack. Make the strip about 4 inches wide, covering the tape completely. Get it smooth and flat. Let it dry to the touch.
5. Lightly sand the strip of joint compound with your drywall sander, getting off any ridges or bumps and smoothing the edges. Reapply a second coat of compound, making it slightly wider than the first one (6 to 8 inches wide). Let it dry.
6. Sand the second coat. Lay a third coat, very thin and wide (about 10 inches across). Once it dries, sand it smooth, then repaint.
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