Bubbles sometimes form on the paint film that look like blisters. These result from the localized loss of adhesion and lifting of the paint film from the underlying surface.

What Causes It?
Applying oil-based or alkyd paint over a damp or wet surface can lead to blistering.Moisture seeping into the home through the exterior walls also causes blistering, though this is less likely with latex paint.Latex paint that is exposed to high humidity or moisture shortly after paint has dried, especially if there was inadequate surface preparation, may also be susceptible to blistering.How to Solve It
If the blisters do not go all the way down to the substrate, remove them by scraping and sanding. Then, repaint with a quality acrylic latex interior paint. If the blisters go down to the substrate, you will need to remove the source of moisture, if possible. Repair loose caulking and consider installing vents or exhaust fans. Remove blisters as noted above, remembering to prime before applying the topcoat.

Note: Images provided by The Rohm & Hass Paint Quality Institute.