I rebuilt my burnt El Camino some years ago. I was told by a buddy that was a professional painter for many years that metal that has been exposed to fire needs to be 'treated' or 'sealed' before it is painted.
He would not guarantee the finish on the front fenders/hood so I replaced them.
So the best way around that is not to use that metal at all.
If they are real cheap I might take a chance using a good primer then a good finish coat of paint.
Chicks dig me.....Fish fear me
Getting older, I have developed some special skills. I have the ability to cough, sneeze, fart and pee at the same time.
"If we cannot afford to take care of Veterans, then we should stop making them."
David C. Wilker Jr. USAF (RET)