Most often it is not the outer surface that is rusting…especially in those fender arches on pickups… its the inner lip and or multiple layers coming together…anything that can catch and hold debris will act like a rust sponge. They usually rot from the back out to the front not front to back. I think we have all seen this…perhaps misinterpreted what was happening but I bet we have all seen it. Starts with those Tell tale Hell Bubbles in the paint…then you know you’re in for it…
Blackbird