Your Heritage Paint Project
Bubbles & Blisters
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
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.