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Prep for painting of "chem-treated" galvanized steel

My company supplies a product made of chem-treated galvanized sheet steel. Occasionally, we are asked by field personnel how to prep this surface for field painting. What do we tell them? All we know is that paint doesn't stick to a chem-treated surface. Is there some kind of simple etching wash they can perform, or is sandblasting required?

P J Moran
Engineer - Grapeland, Texas
2006

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Hi, PJ. Best choice is to use galvanneal steel instead because it is designed for painting. Second choice is to use plain galvanized steel rather than chem-treated. The IHGA offers a booklet, free for the download, entitled "Duplex Systems: Painting Over Hot Dip Galvanizing" at
www.galvanizeit.org/showContent,47,79.cfm

Some claim that washing with vinegar ⇦in bulk on eBay or Amazon [affil links] will etch it for good adhesion; others that scrubbing with TSP (trisodium phosphate ⇦this on eBay or Amazon [affil links] ) and a tampico brush is the best approach. In any event, you must use a self-etching primer designed for galvanized steel. Good luck.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2006

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Ed. note Sept 2013: that page no longer exists at galvanizeit.org, but you can look around the site for something similar.

I suggest to slightly etch the surface with diluted phosphoric acid (10-20%). There are products in the market based on diluted phosphoric acid, which include some surfactants that help wetting the surface.

Barry Groeneveld
- Heijningen, The Netherlands
2006