Dirty Surface

• Any foreign substance on the surface of the plaster that you paint over is a potential blister, which includes everything from dust and dirt to remnants of an old coat of paint. As the new paint dries, it shrinks. On a clear surface this creates a tight, even seal with the plaster. If the surface has anything on it, though, the shrinkage will cause the contaminant to pull away from the surface, leaving you with a blister. Clear the plaster of all debris before beginning.
  
  

Heat

• Exposing the drying paint to heat or painting on a warm surface can cause bubbles. Direct sunlight, for example, can cause the outside layer of paint to dry much quicker than the inside. The heat of the surface then vaporizes some of the chemicals of the interior layer of paint, but with the outer layer dry, the vapor has nowhere to go. Avoid these bubbles by painting the surface when it is cool and painting when the surface is not in direct sunlight. Also remove any heat sources from the area if possible.
  
  

Moisture

• Painting a damp surface or exposing it to moisture before it is fully dried can also cause blistering. When the paint dries, the moisture keeps it from adhering to the plaster. These bubbles are often water filled as the moisture is trapped underneath the coat of dried paint. Moisture can also cause the paint to dry unevenly, which means that, while part of the coat is drying, it is shrinking and pulling away from the parts that are not drying as quickly, leading to an uneven bubbled appearance.
  
  

Type of Paint

• Even if you avoid moisture when painting, the type of paint you use may be susceptible to bubbling and blistering even after drying, which is due to the natural moisture that seeps through most surfaces, especially those on an outer wall of the home. Oil-based paints are used for their water-tight seal but also trap this moisture in. When the moisture seeps through the surface, it can cause bubbles in the paint. Latex-based paint breathes more and is less prone to moisture-caused bubbles.