Foaming or cratering is the formation of bubbles and results into small, round concave depressions in a paint film, during paint application and drying.
Several things can cause foaming or cratering:
Shaking a partially filled can of paint.
Use of low quality paint or very old latex paint.
Use of a roller cover with wrong nap length.
Excessive rolling or brushing of the paint.
Applying a gloss or semi-gloss paint with a long nap roller.
All paints will foam to some degree during application; however, quality paints are formulated so the bubbles break while the paint is still wet, resulting in good flow and appearance. Use a quality synthetic roller sleeve (5mm nap for gloss & semi gloss and 12mm nap for low sheen and flat finish).
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