Blisters in spray paint occur when a layer of paint is laid too thickly or is subjected to adverse conditions. The outermost later of paint dries before the volatile solvents underneath can evaporate. The continued evaporation causes blisters, or air bubbles, to accumulate under the dried layer of paint. Surfaces are marred by this blistering and require refinishing. Additional time to sand and repaint is then required. Avoid blisters when spray painting metal by adding paint in thin layers, away from direct light and on a clean, dry surface.
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Clean and dry the metal to be painted before you start to spray paint. Blisters in paint can be caused from water evaporating off of a metal surface that was spray painted while it was still damp.
Avoid working in direct sunlight or in extremely hot areas. Exterior heat causes the outermost surface to dry at a very fast rate. Blisters result from this layer drying before those below it. Paint in a well-ventilated (but not windy) area and keep painted objects out of direct light until they are completely dry.
Paint in thin, even coats. Applying a thick layer of spray paint to metal in a hurry can cause blisters as solvents from the layers underneath struggle to escape through the thick coating on top.