Hey guys. We are doing a restomod build on an '81 Bullnose and as part of the project we want to treat the light surface rust on the underside of the bed and coat it with POR15. I'm having a hard time figuring out if the bed was coated with something before we got the truck. It looks like there is some sort of undercoating (or mill scale?) that is starting to peel off in a few spots. It would be odd to me that the coating itself would rust, but I guess it's possible. What's also odd is that the rust stops about 1-1.5" from the horizontal rails that support the floor of the bed. I used a light sanding pad on a small part of the rust-looking portion and it looks to me like it's some sort of coating. If someone has some insight into whether this is a factory treatment and why it looks like it does, then that would be much appreciated! Knowing exactly what's going on here will help me proceed with the ideal surface prep.

Thanks,

Chris