The cheap Tractor Supply paint on my trailer has been holding up quite well, and that's with little to no prep! I do wish I'd have done Gloss rather than Matte though.
When my father-in-law added some expanded metal to my trailer to make it more versatile i covered it all with roll on Rustoleum enamel...and a couple of spray cans.
Now I try and touch it up with Rustoleum rust converter (or something) to try and help keep the rust at bay. The boards...well they just rot