There are two main types of blisters-those caused by heat and those caused by moisture. Painting in direct sunlight on a surface that is too warm causes heat blistering. The film dries too rapidly, and trapped solvents later vaporize, bringing pressure against the topcoat creating blistering. This is more common when using a dark color coating, since darker colors absorb the heat more readily than lighter ones.

Blistering can be caused by moisture, particularly in winter months. Interior moisture in tightly constructed homes is a major cause of exterior paint blistering. Moisture building up inside the house escapes through the walls because there is no place else for it to go. In the summer the sun heats the siding, and the water trapped behind the paint film is vaporized. The resulting pressures cause blistering.

The use of alkyd or oil paint finishes in extremely humid environments that have widely ranging temperature variations can also cause blistering. Alkyd finishes are sealer-type finishes that will not allow moisture to vaporize and escape through the siding's surface film rapidly.

SOLUTION: First, determine which type of blistering you have. Break open one of the bubbles. If bare wood shows, the blister was probably caused by moisture; if another layer of paint shows, a heat blister is the likely culprit. In either case, sand and scrape the peeling paint to bare wood; prime exposed areas and repaint. If large areas of the painted surface have blistered and need to be removed, high pressure washing or the use of a heat gun will speed the removal process.

In heat blistering the prime coat is generally not affected; and you can repaint, without priming, when the sun is not shining directly on the surface.

If the blistering was caused by moisture, a number of solutions exist. Repair loose caulking and install vents or exhaust fans if necessary. If the home has lap siding, siding vents can be installed under the siding in areas where blistering has occurred. This will allow moisture from the inside of the house to evaporate before penetrating the wood siding, solving future blistering problems.

If the entire house is re-primed but stands without the finish coat for an extended period of time, more blisters may develop. These should be scraped smooth and spot-primed before applying the finish coat.