Por 15, rust oleum, tractor implement paint, what to use on rusted metal

Yep I did find out from a rep at the company that I planned on using their epoxy primer on the exterior. He told me if I used phosphoric acid to neutralize the metal DO NOT USE their epoxy primer because of adhesion issues. Just sand, scuff, and spray. He said their epoxy primer already has a small amount of acid in it and it will lift the coating left from the phosphoric acid and cause the paint to peel.