FWIW: On the inside of my fenders was a thick layer of roofing/road tar. I think it was a common fix for preventing stars in the paint and the like
Kept things quiet on gravel roads and provided metal and paint protection at the same time.
It was still good when I stripped it off the metal was like a baby's butt and without a doubt the hardest thing to remove on the entire truck. I'd take having to remove five rusted broken bolts over cleaning down to metal one tarred fender anyday.
I wonder if rubberized spray on bedliner will hold up as well for as long...
