Along the upper wheel lip of the bed they used a spray-foam like material and this is where the rust is coming from after it gets wet and rusts from the inside out. Mine has started and will most likely have new bed sides installed.

2003 Dodge 3500 Diesel

1964 Plymouth Savoy White 2dr sdn. 512 stroked, Dominator, New Best 9.75 @ 138.50mph
2019 Challenger Scat Pack 1320 White Knuckle-K&N Cold air filter, Billit Oil catch can, drag radials stock 11.911 @ 114mph