Nothing is as discouraging as finishing a painting project, only to have paint blisters or bubbles appear in the new finish. This type of problem can occur in any type of paint, but latex and acrylic formulas are more susceptible. Bubbling and blistering occur indoors and out, often with no apparent cause. Proper preparation is the single most important thing you can do to avoid bubbles.
What Causes Paint Blisters and Bubbles?
Close-up view of blistering paint on wood siding.
Although the end result is the same, blisters and bubbles are actually caused by two different things. Blistering occurs when the upper layer of paint loses adhesion and separates from older layers. Typically, this happens after the paint has dried completely, and there are no good guidelines for determining if or when it will occur. The problem can crop up within a day, or not until a year later.
The most common cause of blistering is applying paint to a damp or wet surface. Allow the substrate to dry completely before priming and painting. If there is a continual moisture problem (in a leaky basement, for example), it must be corrected before painting.
It’s also important to let the paint dry before exposing it to wet conditions. The weather channel is your friend! Avoid painting within four hours of a rainstorm; high humidity often causes blisters (sometimes water-filled) to form. If the surface drys out quickly, sometimes these will disappear. If a bubble remains after the surface dries, however, you’ll need to scrape it off and repaint.
In other cases, blistering is caused by applying paint to a dirty or incompatibly finished surface. As the paint dries, it shrinks slightly, pulling up poorly adhered sections of old paint. It’s important to thoroughly clean walls (both interior and exterior) before painting. Never apply oil-based paint over a latex base. It is possible to use latex paint over oil based primers and paints, but it’s better to stick with one type of formula. If your siding has been treated with linseed oil in the past, prime and paint with oil-based products.
Bubbles form as the result of a chemical reaction. This problem is far more common when paint is applied to a hot surface. Essentially, the top layer of paint dries, forming a barrier that prevents the solvents from escaping easily. At the same time, the hot surface causes the solvents to vaporize. These expanding gases put enough pressure on the paint to force it away from the substrate, creating a bubble. This occurs early on, during the initial drying phase.
Avoid painting in hot weather (above 85°F) or in direct sunlight. If these conditions can’t be avoided, use a paint conditioner to slow drying. Flotrol is formulated for water-based paints, and Penitrol is a good option for oil-based ones. Avoid using mineral spirits or water for this purpose; excessively thinned paint only compounds the problem.
As with any paint problem, the best defense is proper preparation. If you’re already faced with a paint blistering or bubbling problem, it can be corrected by removing the damaged paint, smoothing the surface and repainting. Just make sure to correct the underlying cause of the problem, or it will recur.