Painting Hot-Dipped Galvanized Steel

Q. I work for a steel fabricator, and we recently received a request to bid on an outdoor structure made from hot-dipped galvanized steel, which we have not worked with before. The customer’s specifications require the structure be painted with a two-part epoxy primer, and then he will apply his own topcoats in various colors. I understand there are a number of cleaning methods and pretreatments available for preparing hot-dipped galvanized steel, but my sources do not recommend these with confidence. Do you know of a successful and fool-proof method? P.P.

A. If I were you, I would turn to other sources for painting advice. Your prospective customer knows that a finish system consisting of a two-part epoxy primer topcoated using a weather-resistant enamel will provide outstanding corrosion resistance to his steel structures. The problem with painting galvanized steel products, including hot-dipped, is the waxy or oily material applied at the mill. In outdoor and some indoor-exposure conditions, galvanized steel will form a loose coating of zinc oxide on its surface. The waxy or oily material is applied at the mill to prevent this “white rusting,” but it forms a barrier coating that must be removed before painting. Pretreatment procedures to remove these waxy or oily materials would include use of aqueous cleaners, solvent degreasers and phosphate coatings.

Zinc phosphates used as pretreatments for galvanized steel prior to painting will require equipment you may not have in your plant. In that case, you can remove the waxy or oily material by simply applying a hot aqueous solution of an alkaline or detergent cleaner to the hot-dipped galvanized surfaces. They can be applied using a steam spray cleaner or power wash sprayer.