Water penetration into the paint film of car bodies causes cosmetic corrosion. The circumstances experienced by driven cars can be classified into three groups, corresponding to high, medium and low water migration, respectively. In the first group a lot of water enters into the paint/substrate interface and lifts up the film. The stability of the phosphate layer influences the adhesiveness. In the second group salt water brings about alkaline blister formation. The corrosion resistance of the substrate against alkali in cathodically polarized state determines the paint adhesion. The third group corresponds to the scab corrosion, which can be suppressed well by zinc coating layer.