Using the wrong type of roller can cause foaming of a paint film. Sponge or foam rollers should be avoided with water-based paints. Long pile rollers must be wetted out properly. Foaming happens when air gets into the wet paint film to create air bubbles. When these bubbles burst craters are left on the film surface. Sometimes these craters dry to give an even film.

Putting it right

You will need to rub down using 'wet and dry' abrasive paper using water or suitable solvent. The surface should then be rinsed and allowed to dry before repainting.