The material you see above the wheel well after you remove the quarter is what causes all pickup trucks to rust above the rear wheels. It's a spray foam used for sound deadening. When it cures it gets hot and eats away at the protective film on the sheet metal. The F150s are aluminum so you don't get it but most other manufacturers it's a matter of time until it begins to happen.
The material you see above the wheel well after you remove the quarter is what causes all pickup trucks to rust above the rear wheels. It's a spray foam used for sound deadening. When it cures it gets hot and eats away at the protective film on the sheet metal. The F150s are aluminum so you don't get it but most other manufacturers it's a matter of time until it begins to happen.
This looks difficult