The most common causes are moisture beneath the paint film and paint applied over dirt, oil, grease or onto a poorly prepared, powdery surface. Water-based coatings may blister if applied over surfaces previously painted with solvent-based paints or primed with a solvent-based stabilising primer that isn’t thinned prior to application. To remedy the problem, scrape back all poorly adherent coating to a firm edge and abrade shiny surfaces with abrasive paper.