Paint Or Undercoat?

A few years ago, just for chits and giggles, I had a hood, with lots of surface rust , even some pitting. I da'ed it, applied Ospho ( phosphoric acid sol.), let it dry, and applied with a foam brush Rustoleum or tractor enamel ( same stuff??).... It sat out in the weather 2-3 years. Then I went to d a off that, I could hardly get it off. Yes the UV rays had fadded it. Yes I had a reaction in places where the enamel wrinkled some. Still could hardly get that off too!!!!
Two things I learned. Use a good primer (epoxy), and UV rays is hard on NON good auto paints!