Bubbling or blistering in newly applied paint is an indication of a problem with the bond between the new coating and the surface. This is usually a problem with latex paints more than with alkyd or oil. In addition, shiny finishes like semi-gloss and eggshell are more likely to bubble than flat latex.
The paint bond can be ruined by a variety of causes. Dust, oil, water or any number of other contaminants can be covering wall, ceiling and trim surfaces. Moisture in concrete and masonry walls, wood, drywall or plaster will also cause this problem. Failure to sand shiny, slick surfaces or failure to prime them may also be to blame for a bubbling finish. When doing new wall repairs, failure to remove all joint compound dust may cause blisters and bubbles.
In some cases blistering may not appear in a new paint coat for a couple of hours or not until the next day. In other cases it will appear immediately. The blisters may shrink as the coating dries and disappear all together, this will sometimes be the last you see of the problem. In other cases the bubbles will begin to crack as they dry further and over time, the paint will peel.
The best way to cure this problem is to prevent it before it happens. Trying to fix the bubbling by scraping and patching immediately after it occurs will not work. You will have to wait a couple of weeks for latex to cure and a couple of days or more for alkyd and oil to dry, before doing repairs.
To avoid bubbling in a new paint coat, prepare the surface properly. First, look for stains and dirt on the surface you're working with and wipe them off if possible. If necessary, wash the surface with soap and water or mineral spirits to remove any contamination. Rinse off any soap or other cleanser and let the surface dry completely before continuing. Seal all other stains with primer-sealer and let it dry completely before doing any repairs. Be sure to remove any sanding dust from the repair process using a damp rag or sponge and wipe gently to avoid marring the newly smooth surface. Prime the new joint compound with flat latex paint or latex primer and let it dry.
Be sure to let new paint dry completely before trying to make repairs. This can take up to a couple of weeks for water-based coatings. Remove all loose and damaged wall material. Pop the blisters with the edge of a putty knife and slip the blade under the paint coat to peel all the loose material away.
Remove the dust and debris from scraping and then seal the surface with a primer-sealer. Stir the sealer well or shake the can to be sure it is mixed thoroughly. Apply a thick coat over the whole area with a brush and let it dry for about 30 minutes. Apply a second coat if the surface has been wet or you are repairing bubbling in newly applied paint. Let the final coat of primer set for an hour or more to be sure it is completely dry.
If the surface you are dealing with has a chronic bubbling problem or you are repairing bubbling in newly applied paint, using setting-type joint compound for this repair will minimize the chance of the blisters recurring. Ready-mixed compound tends to add too much moisture to the surface and can induce bubbling in the finish.
Immediately skim the excess mud from the surface to leave a thin covering over the damage. Hold the knife at about a 30° angle and press down hard, but not hard enough to dig into the surface, as you skim from one side to the other without stopping. Scrape the collected mud from the knife onto the rim of a mud pan and continue skimming the remaining mud in parallel strokes, cleaning the knife after each stroke.
Ignore any ridges left between the rows by the edges of the knife blade, these will be removed later. Around the perimeter of the patch, skim the mud as thin as possible to help blend it into the surrounding wall. Let the mud set for about 30 minutes.
When the mud is dry, scrape off any ridges using the joint knife to cut them off with upward strokes of the blade. Also lightly sand the whole patch if necessary to smooth out any other bumps or protrusions before applying another coat. At the edges of the patch, sand to feather the mud with the surrounding surface.
Wipe the sanding dust off with a damp rag and apply a second coat of mud, skimming perpendicular to the first coat. If you skimmed the first coat horizontally, apply the next with vertical strokes. This will help to level out the surface.
Let the second coat dry and if necessary, apply another coat to build up the patch until it hides the damage and smoothes out the wall. As before, skim the mud stroking perpendicular to the previous coat.
Let the patch dry well and then sand it using light pressure, in wide back and forth strokes, to smooth and blend the surfaces. Along the edges, sand harder to help feather the patch into the surrounding surface but avoid digging into the wall. Wipe the sanding dust away with a damp rag or sponge being careful not to dig into the new joint compound.
Let any moisture from the rag dry before priming. If you are repairing bubbling in a new paint coat or a water-damaged surface, prime with the same primer-sealer used before to be sure the problem doesn't recur. Let the primer dry for an hour or so before touching up the finish.