Over mixed joint compound can leave pitting bubbles on the wall.
Joint compound is the smooth "mud" used by drywall contractors to fill the seams between drywall panels. Applied while wet with large taping knives, the compound should smooth out evenly, creating an even plane. Sometimes, after applying a layer of compound, bubbles appear. This may be due to the consistency of the compound or to problems with the wall surface.
Pitting
Small surface bubbles in the joint compound, known as "pitting," may occur if you don't use enough knife pressure when smoothing out the compound. Bryan Tradem, author of "Working With Drywall," informs readers that over mixing the joint compound can also result in pitting.
Tape Bubbles
Paper drywall tape, used to cover the seams between drywall panels, must be thoroughly "bedded," or bubbles may pop up on the wall. Correct bedding requires an even layer of wet joint compound on the seams before positioning the drywall tape. Using too little compound, or positioning the paper tape after some of the compound dries, can result in tape bubbles.
Existing Walls
Joint compound goes on wet and the moisture seeps into the wall beneath. Old walls may have layers of wallpaper and paint. If the wallpaper is firmly stuck to the panels beneath, you may be able to skim coat the wall with thinned joint compound. Test a small area first by skimming only a couple of square feet and letting the joint compound dry. If the moisture from the compound loosens the wallpaper adhesive, bubbles will form.
Smoothing Bubbles
Using premixed joint compound reduces the risk of air in the compound that results in pitting bubbles. If the compound creates bubbles, keep taping but go back to each seam a minute or two after you finish it to give the bubbles a chance to surface, and smooth over the seam again with the taping knife.
To fix paper tape bubbles, cut the tape, peel back the edges, put a little compound beneath the loose area and continue taping.
If bubbles from taping over the old wallpaper appear, you'll have to strip off the wallpaper before you can skim the wall.
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