Personal experience... marine enamel or a really good industrial enamel. I had a CDN M101 that was crudely painted by the PO on the outside only with Tremclad. Inside was OEM OD Green still. Prep is key, and preventing rust of course.
My method.
Sand and or grind back all rust without taking away too much metal. Sand entire surface with a decent grit (maybe 180 or 220) and when done, wash down with a good wax and grease remover to remove all contaminants. I used POR15 Metal Prep over the entire tub and frame because there was a lot of bare spots. I then painted the entire trailer with POR15, 1 light coat. I then sanded it lightly with 220 to give it some tooth and re-wiped it with wax and grease remover.
Top coat I chose was Interlux Marine Enamel in bright red. There are numerous colours and options...I used single component (non-activated) and a small foam roller and brush. The finish was amazing after 2 coats! If you stood back, it looked like I sprayed it. Once fully cured, it was as hard or harder than automotive paint and looked the same 5 years later. Hardly a scratch.
Prep is key, and using components that are compatible and following the directions. Worry about timing and temps... if it says do not re-coat before 2 hours or after 5 days....DON'T. It will wrinkle and you will have to strip the whole thing. Bonding can also be an issue, so ensure you are using stuff that works nicely with each other.
A cheap paint you can buy is Industrial Enamel for machinery. It comes in decent colours, is usually VERY colorfast (wont fade) and is abrasion resistant. I'd be wary of some of the 2 component stuff, it can contain some pretty nasty chemicals (Isocyanate and other stuff like Toluene). Oh and wear a proper respirator with proper cartridges. Can be picked up at Home Depot cheap.
5 years old...
GregB_CDNM101_Sized3 by
Gregory Bird, on Flickr