Depends on when...usually solvent or some contamination in the paint or on the metal or primer, paint underneath was not compatable, rust, etc. - Bill
a pin hole (tiny rust hole) will cause paint to bubble. also, if this is clearcoat/base paint, then if the proper curing/drying process was not followed or too many coats of either were laid down too soon, the the gases that are produced as the paint cures can cause bubbling.
