PURPOSE: To improve the adhesion of a Zn alloy electroplated steel sheet to paint by electrolytically cleaning the plated surface with a slightly acidic or neutral aq. soln. and sticking a specified amt. of a phosphoric acid compd. on the plated surface.

CONSTITUTION: A steel sheet is electroplated with a Zn alloy, the plated surface is electrolytically cleaned with a slightly acidic or neutral aq. soln. and a phosphoric acid compd. is stuck on the plated surface by 0.01W50mg/m2 (expressed in terms of P). Any ordinary electrolytic cleaning soln. of ≥4 pH may be used as the electrolytic cleaning soln. Soluble hypophosphorous acid, phosphorous acid, orthophosphoric acid, an alkali metal or alkaline earth metal salt thereof can be utilized as the P feeding source.