The primer of double electrodeposition coating is formed by electrically depositing a thermosetting quaternary ammonium-epoxy resin adduct on the metallic material, and then the topcoat, a self-crosslinkable acrylic resin, is formed upon the wet primer, by electrodeposition again. It is observed that the quaternary ammonium-type adduct, used as the primer, can lead to thicker topcoat film, than the tertiary amine-type adduct. Meanwhile, a higher applied voltage, a longer deposition time or a higher resin concentration of emulsion for the topcoat electrodeposition can also increase the topcoat thickness. Moreover, when the primer contains the conductive compound, such as carbon black powder, a thicker topcoat can also be formed.