ACE hardware has some spray paint called Rust Stop. I’ve been using their satin black on several thing the last few years. It goes on really smooth and looks great.
Walmart has always been my choice of stores for Rust-Oleum...get some gray primer and soak the inside of the tube...wipe off any excess and primer the outside. I'd do a couple coats of flat black or semi and a coat of bedliner. Harbor freight carries bedliner.
If you can weld consider cutting off that latch...far better arrangements out there...