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If the crack extends through the seam’s paper tape, or if the tape has pulled loose from the wall, use a razor knife to cut the tape about 6 to 12 inches from both ends of the damage (image 1 and 2). Remove the tape but be careful not to tear away the drywall’s paper covering. Scrape away any loose compound, and use a razor knife or drywall saw to expand the crack through the wall surface into the stud cavity (image 3). Avoid removing solid, well-adhered compound beyond the crack itself.
Fill the crack with new drywall compound, and apply a thin coat of compound to the wall surface where the old tape was removed. While the compound is still wet, place a strip of fiberglass tape over the seam, bridging the gap between the ends of the existing tape (image 4). Use a putty knife to gently flatten wrinkles and to bed the tape into the compound.
After the compound dries, add a second thin coat of compound over the taped area. Cover the tape and taper or “feather” the edges of the new compound onto the surrounding wall surface (image 5). Drywall compound needs to be applied in multiple thin layers because thicker layers are too difficult to smooth out and will eventually cause cracking.
When the second coat is thoroughly dry, sand lightly to smooth out any bumps. Next, use a wide (8- to 12-inch) joint-compound taping knife to completely cover the patch with a third and final coat. Try to blend this coat as seamlessly as possible onto the wall surface. After it dries, sand lightly, wipe away dust and repaint the entire area.