The repair techniques on this page can be used to repair wall and ceiling cracks in plaster or drywall. However, major plaster cracks caused by extremely loose and crumbling plaster can not be repaired using this technique alone. For most cracks, taping is an effective and permanent solution, but if your plaster is cracking because it is loose, the lath and brown coat must be stabilized to cure the problem before repairs are made.
The first step is to remove any loose material from the crevice. Run a putty knife through the crack several times to loosen any crumbling wall material. Also scrape away any peeling paint along the edges. Clear away all dust from the scraping, and prime the surface to prevent bubbling.
Bubbling may develop in the finished repair if contamination prevents bonding between the surface and joint compound. This is a common problem when water has damaged the surface. If there's any contamination, sealing it with a shellac-based primer-sealer will ensure that your repair efforts aren't wasted. You can use a spray can of sealer on lightly damaged surfaces or brush primer on heavily damaged surfaces to be sure of good coverage. Let the sealer dry for 30 minutes to an hour before proceeding.
Apply fiberglass mesh wall tape all along the crack to reinforce the wall. If you need to cut the tape to follow an irregular path, overlap the ends of each new piece by a half inch or so.
If wall surface is loose, further support will be required to help prevent future damage. In some cases, it may be possible to drive screws into framing around the damage to stop movement. This is not always possible because framing studs and joists may be too far away from the crack to be of use. In these situations you can try filling them with foam insulation to stabilized a wall.
Use back and forth strokes to cover the tape with a very thick coat of setting-type joint compound. Cover the tape well, while pressing mud in to fill the crevice and the fibers of the tape.
Hold the joint knife at about 30° and press down firmly while drawing the blade along the tape to remove the excess mud. Avoid using too much pressure that would dig into the tape fibers, but enough to leave a thin coat of mud behind. Don't try to completely hide the tape with the first coat, it will require a few more to cover the fibers and smooth out the wall.
Allow the mud to set for about 30 minutes and use medium sandpaper to sand very lightly over the raised tape fibers to knock them down. Don't dig into the tape just remove the roughness with a swipe of two with the paper. Use a damp rag to wipe lightly over the repair to remove the sanding dust.
Cover the dry mud with another coat overlapping the edges of the first a bit. Stroke back and forth across the tape to cover it and the dry mud with a thick coat. Immediately go back and stroke parallel over the tape to skim off the excess mud. Stroke along the edges first and then smooth out the center to leave a thin covering over the tape.
When the mud dries, sand again to smooth out any rough spots and then wipe the dust away with a damp rag. Apply a third coat of thick mud over the whole patch overlapping the edges of the second coat. Skim off the excess mud stroking across the tape this time to leave a smooth finish.
When the mud dries sand the whole patch paying extra attention to the edges feathering them into the surrounding wall surface. Go lightly over the center portion of the mud to avoid digging in and removing too much covering. Use a dry brush or vacuum to remove the excess dust and then wipe lightly with a damp rag to remove any dust that would inhibit the paint touchup.
If you are using flat latex finish paint, you can use it to prime the repair. If you are using semi-gloss or other shiny paint, prime with flat latex or a latex primer first.
Use this process to repair recurring cracks in gypsum board plaster and drywall where you can't drive screws to stabilize excessive movement. This technique makes use of latex caulk to hold the gypsum in place and stabilize movement. When the caulk has set it will flex with the wall material to minimize future cracking.
After clearing away the loose wall material use a spray bottle of water to soak the plaster and gypsum along the crevice until it won't absorb anymore water. This will help to ensure a lasting bond between the wall material and caulk filler. Fill the gap with plenty of caulk and smooth it into the surrounding wall material with your finger and a wet rag. Wipe any excess off the finished wall surface around the repair area. Let the caulk dry and set for an hour or more before taping and repairing using the method above.
If you're dealing with loose or bubbling drywall tape, use a sharp drywall knife to remove the damage, cutting cleanly around the edge of bubbles. If the tape is coming off completely cut it off squarely, just outside the damaged area and pull the loose paper off.
Prime the exposed seam and surrounding wall surface with shellac primer and let it dry completely. Apply fiberglass tape over the whole thing and then skim over with joint compound. Follow with as many coats as needed to hide the tape and finish using the same method described above.