Blistering:Bubbles resulting from localized loss of adhesion and lifting of the paint film from the underlying surface.
Painting a warm surface in direct sunlight
Application of an oil-based or alkyd paint over a damp or wet surface
Moisture escaping through the exterior wall (less likely with latex paint than with oil-based or alkyd paint)
Exposure or latex paint film to dew, high humidity or rain shortly after paint has dried, especially if their was inadequate surface preperation
If blisters go down to the substrate, try to remove the source of moisture. Repair loose caulking; consider installing an exhaust fan. Remove blisters
If blisters do not go all the way down to the substrate, remove them by scraping, then sanding; prime any bare wood and repaint with a quality exterior paint.
Note: Images provided by The Rohm + Hass Paint Quality Institute