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.
Do you know what was good about the good old days? I wasn't good, And I wasn't old!
Senators and Congressman should wear uniforms like NASCAR drivers so we could identify their corporate sponsors....