Undercoat paint?
I have light like dustings of rust in spots but being freshy painted would be nice very thing else is mint. Think of it like this oh damn look at that sexy girl (truck) then they turn over or around and you're like Daaaaayyyuuummn wtf
It's not the cheapest and is a PIA to work with, but POR15 is about the best out there to prevent any rust. This is the stuff you can hit with a hammer and doesn't chip. Then just put a good top coat over it (it doesn't like UV light) and you're good for the life of the truck. I plan on taking the bed off of my truck this summer and doing the frame and bed underside.
Waiting for some of the Southern guys to chime in and laugh about how they don't have rust ever.
Waiting for some of the Southern guys to chime in and laugh about how they don't have rust ever.

