If the panel is new or bare metal you are trying to protect short term and will be welding on it then use weld through pmr. If you are preparing the panel for finish work then you can use self etch pmr and then cover it with a urethane pmr. Another option is to shoot epoxy pmr over the bare metal. The epoxy is probably the best sealer, but it is temperature sensitive and requires you to follow a time schedule. The epoxy is probably going to offer the most protection. I would shoot urethane over the epoxy within the time the data sheet specifies. Body shops prefer the self etch and urethane, as you can begin working them in a few hours. I used self etch then a high build urethane (UPOL brand) on mine, and we did epoxy followed by the UPOL on my friends Cutlass. Both with outstanding results. We really took a liking to the UPOL, you can block the heck out of this stuff!

I am not sure about cold galvenizing, but I know if you weld galvenized metal it is very dangerous.