[lizziemeister'sV6]

Could be that the tar coating you're talking about was applied as undercoating when the truck was new. A good idea that worked well until the coating aged and cracked then allowed road grime to get under it and cause rust. We have all experienced the outcome of salt/dirt that found pockets and seams to rest in over the years - some of us more in the "Rust Belt" compared to the sunny southern states.