A primer coating composition comprising (1) a mixture of more than 50% by weight of aluminum powder and less than 50% by weight of zinc powder, and (2) at least one C.sub.1-4 -alkyl silicate, the weight ratio of (1) to (2) being 60:40 to 95:5. The composition is employed in a coating method which comprises applying a primer dispersion to the surface of a substrate to be coated. The primer disbersion is a dispersion of a mixture of more than 50% by weight of aluminum powder and less than 50% by weight of zinc powder in a solution of at least one organic solvent-soluble C.sub.1-4 -alkyl silicate in an organic solvent. The weight ratio of the metal powder to the alkyl silicate is as indicated. The coating is baked and a finishing powder paint is coated on the resulting primer. The finished coating is then baked.