Most any 2K primer will keep out moisture. I feel Epoxy is the best, but honestly, I have never seen any real study to prove that.
I would etch prime it, then epoxy. The real trick is to be sure it is TOTALLY covered. People have the habit of not totally covering every single square inch with the same film thickness of primer. People will not give it the respect it deserves as they spray it leaving it too thin and it doesn't protect it from the weather.
I have a vintage gas pump in my yard. It was sand blasted, etch primed, then urethane primed. It has not seen one day of cover in 8 years. There is ZERO sign of ANY rust what so ever. So, yes urethane primer will protect. Of course, the etch makes a big difference. But in this case the etch is not exposed at all so at this point it is doing nothing. If it was scratched, then the etch would come into play.
