How to Repair a Painted Drywall Seam
A bad drywall seam will bother you every day until you deal with it. Often, the problem with the seam isn't obvious until after the wall is painted, by which time it is more complicated problem to fix. Spreading joint compound over paint can be done if it's a flat, clean paint, but you're usually better off stripping the paint first to ensure that the joint compound will stick. Have a supply of the same paint that is on the walls so you can patch-paint instead of re-doing the whole room.
Sand off the paint from the seam with a belt sander and 80-grit paper. Sand about 6 inches on either side of the seam. Remove all the paint until you reach drywall and joint compound.
Reload your belt sander with a fresh belt of 80-grit sandpaper (the paint will have clogged up the previous belt). Run the sander over the seam to remove the plaster. When you get down to the connector that was used between the sheets of drywall (either mesh tape, or paper tape) pull it off. Make the surface smooth and free of loose plaster.
Lay a strip of mesh tape over the exposed seam. Use your 6-inch drywall knife to spread a thin line of joint compound over the tape, just covering it. Let it dry for a few hours.
Sand down the seam with your drywall hand-sander and screen. Use your 12-inch knife to spread a wider line of compound over the seam, about 6 or 8 inches wide. Taper the line of compound at the edges. Let it dry.
Sand the second coat of compound with your hand-sander, and apply a third coat, using your 12-inch knife. Make the seam about 10 inches wide, tapering it at the edges. Once it dries, sand it very light to get the seam completely smooth.
Repaint the repaired section of the wall.
- Wear a particle mask and goggles during all the sanding.