Painting hot dip galvanized metal is done on the regular basis (zinc chromate primer) but there are long term consequences which need to be understood.
The reason for the use of hot dip galvanizing is the three-fold protective nature of the coating.
1. As a barrier coating, it provides a tough, metallurgical, bonded coating that completely covers the surface sealing the steel from the corrosive action of the environment.
2. Zinc?s sacrificial behavior protects the steel, even where damage or a minor discontinuity in the coating occurs.
3. Natural weathering of the coating results in the development of an additional layer of protection (zinc oxide) on the surface.
Painting a galvanized part affects all three components, but the major effect is to hinder the Zinc Oxide film (materials' primary form of protection) ineffective, if not useless. Any chips in the paint will expose the zinc to oxygen causing it to bubble and peeling the paint. Moisture gets trapped under the "loose" paint, displacing the oxygen needed to form its protective zinc oxide layer and corrosion is off to the races.
