A blister can form when vapor expands between a coated surface and the film that protects it.
What Causes It?
Anything from humidity to construction defects to a lack of effective vapor barriers often can lead to moisture condensation on wood siding, which generates pressure on the wood and its coating. As the surface dries, water evaporates, but the paint film does not recover.
Blisters can also form when painting in direct sunlight with solvent-based coatings on a surface that is too warm. When you apply thicker coats, or re-coat before a previous coat has completely dried, the paint surface can dry before the solvent releases from the underlying film. This traps the solvent, which can expand and cause blisters.
How to Solve It
To remove blistered paint, sand and scrape peeling paint to the bare wood. Prime the exposed areas and repaint them.
For blisters caused by moisture, minimize any future problems by repairing loose caulking and installing vents or exhaust fans. These will allow moisture to evaporate before it can penetrate wood siding.
For large blistered areas, use a high-pressure washer or a heat gun to speed up the process of removing the paint.
Remember that blisters occur only in fresh coats of paint. When they break, they contain no liquid and do not indicate
a defect in the wood. They can be removed by scraping and sanding.
Note: Images provided by The Rohm & Hass Paint Quality Institute.