PROCESS FOR TREATING PLATED STEEL SHEET

摘要

The present invention relates to a process for surface treatment for cathodic electrodeposit coating of a zinc-plated or zinc alloy-plated steel sheet comprising coating and drying a protective coating material on said steel sheet chromate-treated beforehand, to form a protective film before subjecting the sheet to the cathodic electrodeposit coating, which comprises coating a protective coating material having as a principal constituent of film-forming organic component a block isocyanate-containing prepolymer containing in one and the same molecule respectively at least one blocked isocyanate group, hydroxyl group, and tertiary amino group and containing 10 - 100 parts by weight of silica sol relative to 100 parts by weight of said prepolymer, in an amount of 0.3 g - 4 g/m<2>, and then heat-drying the same.