Paint is widely used and very effective means of protecting metals from atmospheric corrosion. Paint protects metals in two ways: by separating the metal from its corrosive environment (such protection is not perfect because paints known today are not completely impervious to moisture and oxygen), and by the use of inhibitive pigments, which provide protection when the paint is penetrated by corrosive elements.
Despite this dual nature of paint protection, even the best of paints, if applied to an improperly prepared surface, may fail to give the desired degree of protection. Too, some corrosion problems cannot be met by paint alone.
In practice, metals are covered with films of materials such as: corrosion products, mill scale, rust-proofing oils, surfacing compounds, dirt, grease, etc. This layer can be wet by paint, but may itself lack adhesion to the metal. Small quantities of oil and grease and even finger-prints, will prevent adhesion...
