Filiform corrosion-resistant primer coating composition and method for preventing filiform corrosion

A method of inhibiting filiform corrosion which may occur on the surface of a metallic substrate, which comprises applying a primer coating composition comprising a film-forming component containing a film-forming resinous material and a hydrotalcite component to the substrate surface to form a coated film layer thereon, the improvement wherein the hydrotalcite component of the coating composition is a hydrotalcite solid solution represented by the following formula (1)

• [(Mg.sup.2+).sub.y 1(M.sup.2+)y2].sub.l-x M.sub.x.sup.3+ (OH).sub.2 A.sub.x/n.sup.n-.nH.sub.2 O
• wherein M.sup.2+ represents at least one divalent metal cation selected from the group consisting of Zn, Cd, Pb, Ca and Sr, M.sup.3+ represents a trivalent metal cation, A.sup.n- represents an anion having a valence of n, and x, y1, y2 and m represent positive numbers which satisfy the following conditions:
• 0<x.ltoreq.0.5,
• 0<y1,
• 0<y2,
• 0.5.ltoreq.y1+y2<1, and
• 0.ltoreq.m<2,
• the amount of the hydrotalcite solid solution being 0.1 to 20 parts by weight per 100 parts by weight of the film-forming component.