There are two main types of blisters: those caused by moisture and those caused by heat.
Moisture blisters can be caused when moisture from inside a building is drawn to the outside walls because it has no where else to go. In summer, the sun heats the siding, and the water behind the film vapourizes.
Blistering can also result when painting over a surface that has had insufficient drying time after exposure to moisture from rain, dew or pressure-washing. Unlike peeling problems, the moisture blisters may actually disappear afterwards and remain almost undetectable.
Heat blisters result when the sun causes the outside of the coating to dry too quickly, trapping solvents or water between the old and new coats of paint.
Determine which type of blister exists.
Break open one of the blisters. If bare wood shows, the blister is likely caused by moisture. If another layer of paint shows, it is likely caused by heat.
If caused by moisture:
i) Locate the source of moisture.
ii) Repair worn or loose caulking.
iii) Install ventilation devices.
iv) Install siding vents under siding boards.
If caused by heat:
i) Do not paint new coat in direct sunlight or extreme heat.
ii) Remove blisters.
iii) Feather the edges with sandpaper.
iv) Spot-prime bare wood with oil-based primer.
v) Apply topcoat.