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.
"Taking care of our veterans is a cost of war. If you can spend six trillion dollars sending people to war, you can spend a few billion dollars taking care of them when they come home"
Sen. Bernie Sanders (I-Vt.)
If what doesn't kill you, makes you stronger, I should be able to bench press a Grand Cherokee by now!