Q: What are the pros or cons of powder coating with zinc primer vs. galvanizing and then powder coating over top?
A: Personally, I like the idea of bare steel (non-galvanized) being properly cleaned and pretreated, followed by a good epoxy primer (doesn’t have to be zinc rich), then a high-quality polyester topcoat. The most critical step is cleaning/pretreating the steel. If it’s hot-rolled pickled and oiled (HRPO), then the cleaning/pretreating step is difficult. The challenge here is to completely remove the “oiled” layer. This can sometimes require a strong acid etch stage in your pretreating scheme.
If it’s cold-rolled steel, then the pretreatment process is much easier: A good alkaline cleaner followed by a rinse and then a chemical conversion like iron phosphate or a zirconate followed by a rinse and perhaps a sealer.
It’s a very good idea to gel, but not cure, the epoxy primer prior to applying the topcoat. This enhances intercoat adhesion by allowing the unreacted chemical groups in the epoxy to react with carboxyl groups in the polyester.
I don’t like HDG (hot-dipped galvanized) because it’s variable, it needs degassing, and pretreatment can be tricky. A really good electrogalvanized surface is a different story, but job coaters probably don’t see electrogalvanizing too often.