Drywall seams, at times, have a tendency to crack. Sometimes it's from a very subtle movement of the wall. Particularly on walls near windows or doorways, any slamming of the door or window can create a movement in the wall that, even though very slight, can create a crack in a drywall seam. Regardless of the cause of the crack, it will need to be repaired. With a basic understanding of the process used to repair these cracks, along with the appropriate tools and materials, you will be able to repair cracks in your drywall seams with little trouble.
Step 1 - Prepare the Crack For Filling
When you have located the crack to be repaired, determine the severity of the crack. If it is wider than a ¼ inch, or if the crack is near a door or window where the wall is more likely to shift or vibrate, you may need more than the standard repair.
Step 2 - Strip the Finish, Plaster, and Tape from the Crack
You will be replacing the existing drywall tape, not taping over it. So, you'll need to remove the tape and plaster. You can best do this by using a scraper. Push the edge of the scraper under existing drywall plaster and force the plaster and paper away from the drywall surface and along the length of the crack. Remove enough of the plaster and paper that you'll have a smooth surface on which to apply the new tape and mud. If necessary, use a coarse sandpaper to remove rough places on the wall surface near the crack.
Step 3 - Apply a Bed of Mud
Mix a small amount of drywall mud with water to thin the mud. Then, load some of this thinned mud on your joint knife and apply it over the crack, by moving your joint knife with the mud on it down the crack. Be sure you leave enough mud on the wall along each side the crack that is as wide as the drywall paper you'll be applying to the crack.
Step 4 - Apply the Drywall Paper Tape
Cut a strip of drywall tape long enough to cover the crack in your seam. Press this strip into place against the mud bed, using only enough pressure against the knife so that the tape adheres to the mud bed but doesn't create a dent. Position your joint knife with the blade horizontal across the adhered tape. With the knife blade at a 45-degree angle, press the knife edge against the tape and draw it down the tape to smooth mud edges and adhere the strip to the mud bed. Smooth away obvious ridges in the mud to make the mud strip as smooth as possible.
Step 5 - Sand the Seam and Reapply Mud
Allow the drywall mud on the seam to dry. Use your sandpaper to smooth away any ridges or bumps. Then, with your knife again loaded with thinned drywall mud, float it down the seam, matching the surface with that of the wall around it. Let it dry, sand again, and you are now ready to apply primer.
WARNING: To avoid getting any drywall mud particles into your eyes or lungs, wear safety glasses and a face mask while sanding (especially if you use an electric sander rather than sanding by hand).