Blistering is the formation of round 'bubbles' of paint film. The cause is always moisture related:
Excessive moisture coming through the substrate. This can happen when humidity seeps from a room with very high humidity, such as a bathroom or laundry, or
Exterior paint is applied over a damp or wet surface. If paint is still in the drying phase, blisters may appear during periods of rain or heavy dew.
These problems are more common in solventborne or alkyd paints due to the vapour barrier that forms as they dry, trapping moisture on the wall beneath the paint. As the paint dries, that moisture will escape, lifting the paint off the wall as it evaporates. Acrylic paint is more permeable - it allows a small amount of water vapour to pass through as it dries and all but eliminates the chance for these types of blisters to form.
However, there is one form of blistering that affects waterborne paints. It occurs when the paint is subjected to excessive amounts of moisture, causing the paint to swell and break its adhesion to the substrate. This may appear worse during wet periods as the blisters fill with water. Blistering due to swelling of the paint is most likely to occur within the first few days on the life of the paint job. After the paint has been exposed to the elements, rain and other moisture will typically have 'leached' water soluble components from the paint.
Paint containing small blisters (less than 6mm across) due to swelling often recovers after drying out. When larger blisters form the adhesion of the paint is often weakened and peeling can be the result.
To avoid blistering, acrylic paints should be applied at specified spreading rates and be well dried before dew or rain strikes. This may well mean stopping painting at about 1pm or 2pm. Problems are more likely on horizontal surfaces on clear, windless, days in autumn.
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