Bubbles, or blisters, can occur in paint on both interior and exterior surfaces. Heat or moisture are the two most common causes. Heat bubbles usually occur when painting in strong sun. A thin surface area of paint dries, and a thinner layer in the wet layer below changes to vapour that cannot escape. The second cause is moisture, from an interior or exterior source, which expands paint like air in a balloon. Removing paint bubbles is relatively simple, but with moisture problems you may need to tackle the cause of the problem rather than just the symptoms to prevent it happening again.
Break open a blister with a paint scraper and see what is inside. Bare wood inside indicates a moisture problem while an under layer of paint inside indicates a heat problem.
Locate the source of any moisture, if possible, and eliminate it. Improperly vented gas fires, faulty plumbing leaks, shower spray and humidifiers may all be sources of interior moisture, while faulty guttering or insufficient overhangs may contribute to exterior moisture according to Michigan State University Extension.
Wait for the surface to completely dry. This will usually take at least two days. Trying to remove bubbles when the paint is still wet will be very difficult, and painting on a wet surface will cause bubbles to form again.
Scrape off bubbles and loose paint using a paint scraper.
Sandpaper any remaining paint smooth with the wood.
Clean thoroughly using a bristle brush and rinse with clean water.
Paint any wood surface with a water repellent. This will stop moisture soaking through the wood. Wipe off any excess with a damp cloth.
Wait for two days, or until the water repellent is completely dry.
Apply an oil-based or latex primer, and wait until it is dry. This will take approximately two days according to House-Painting-Info.
Apply at least one layer of high-quality acrylic latex paint. Several thinner layers will last longer than one thick layer and will be less likely to bubble.
Avoid heat bubbles or blisters by painting on overcast days. Alternatively, follow the sun around the house, always painting on shaded areas. On masonry walls, you can use a pressure washer and should apply a masonry sealer before repainting. Use a quality acrylic water-based interior paint if painting inside rather than acrylic latex paint.(See references 2) Paint with a dark colour paint to avoid heat bubbles.(See references 2)
Unless you remove the source of any interior or exterior moisture, bubbles are likely to re-form.