Why Paint Jobs Fail

BLISTERING:

Blistering is caused by pressure under a non-permeable paint film. This pressure may be developed by vapor or moisture. The vapor may come from wood or solvent entrapment in the paint film. Moisture can come from four sources structural defects, accidental spillage, precipitation and humidity. Permeable (breathable) paints, such as 100% acrylic house paint, allow moisture vapor to pass through the film and thus do not blister.

Blisters caused by moisture under the paint film usually are the result of structural defects in the building or high humidity within the home or building. Unseasoned wet wood will also cause blistering.

Vapor or solvent entrapment will also cause blistering when painting is done in direct heat of the sun. The direct sun heat causes the paint to set quickly. The surface vapor is trapped underneath and causes pressure, forming blisters. This usually occurs in the first twenty hours after the paint is applied. Dark colors are usually the worst.

Modern construction tends to intensify high humidity in the home due to insulation, weather stripping, caulking, storm doors and storm sash; the objective is to keep the heat in and the cold out. During the winter this humidity builds up in the home until it reaches almost 90% in midwinter, January and February being the worst months.

The humidity or moisture vapor explained above must escape through open windows or open ventilators of some type. If it does not, it will force itself through he walls and open spaces around doors and windows, condensing in the outside walls, which are cold. Here it forms water and builds up pressure under the paint on siding or trim, causing blistering of the paint. This can be prevented by installing louvers or ventilators to promote air circulation.

If the siding and trim become saturated with moisture they can be ventilated by installing louvers in the siding at the locations badly affected. Also, wedges may be driven under the bottom edge of the siding to allow air circulation under the siding. Louvers of wedges should be placed about 16 inches apart between rows of the nailing of the siding.

All exterior structural defects should be repaired before painting. Leaks in roofs, gutters or spouting should be repaired. Openings around windows, doors, ends of siding and corners should be caulked or puttied after priming. Every measure should be taken to prevent driving rain from entering under the paint film.