Residual surface contaminants, mainly sulfates and chlorides, resulting from the exposure of steel to industrial and marine atmospheres before painting, can significantly influence the behavior and lifetime of paint coatings. This research studies the corrosion of steel beneath a thin varnish film when variable amounts of FeSO₄ and NaCl are present at the steel/varnish interface. The effects produced by both contaminants when acting separately and in combination are analyzed. In the first case, underfilm corrosion of steel depends on the surface concentration of contaminant. In the joint action of both contaminants a quasi-additive effect is observed.