What are the specific requirements for painting over galvanized steel?
The specifications for preparing hot-dip galvanized steel for painting or powder coating are ASTM D6386 and D7803, respectively. Successfully painting or powder coating hot-dip galvanized steel, also known as a duplex system, does not have to be difficult or confusing. Just like painting or powder coating over bare steel, proper surface preparation is crucial to ensuring effective adhesion. Additionally, slightly different methods may be recommended according to the condition of the galvanized surface (newly galvanized, partially weathered galvanizing, fully weathered galvanizing). Those methods may include solvent, alkaline, or ammonia cleaning, profiling using a media softer than zinc, such as corn cob granules, and conversion coatings. Key to success is notifying the galvanizer the material will be painted after the galvanizing process and consulting with the paint manufacturer to ensure a paint specifically formulated for use of zinc coatings is used. Although the selection of a duplex coating for corrosion protection usually involves aesthetics/color, the net result is a coating which lasts from 1.5 to 2.5 times as long as the sum of the two coatings would if used independently. This is called the synergistic effect and is an excellent reason to specify a duplex coating for projects designed to be maintenance free for many decades. The AGA offers a variety of publications on preparing HDG steel for paint or powder coating.
© 2019 American Galvanizers Association. The material provided herein has been developed to provide accurate and authoritative information about after-fabrication hot-dip galvanized steel. This material provides general information only and is not intended as a substitute for competent professional examination and verification as to suitability and applicability. The information provided herein is not intended as a representation or warranty on the part of the AGA. Anyone making use of this information assumes all liability arising from such use.