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

I am attempting to replace all metal that has any significant amount of rust damage. Some support structures behind the quarters are difficult to get to so I am applying phosphoric acid then I need to paint it. This is the situation I am asking about. These areas won't see the light of day nor were they originally painted. Am I going to extremes that are unnecessary or is it great protection?