I would just wash with muriatic acid, same stuff used in swimming pools, flush with water and a little soda. When dry, give them a coat of epoxy primer, let it flash and spray the color coat of you choice.
I have as sign on the front of my store made of 26 gauge galvanized that was prepped with muriatic acid and painted with oil base paint 10 years ago. Still looks new.
It used to be standard practice to etch galv gutters, ducts and other building stuff with muriatic or vinegar before painting. Besides, epoxy will stick to most anything!
