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.
"Once good behavior becomes the exception and not the rule your society has begun to fail…..we are there." laybackman
Life is like a coin. You can spend it any way you wish...but you can only spend it once.
"If we cannot afford to take care of Veterans, then we should stop making them."
David C. Wilker Jr. USAF (RET)