Very nice work on the trailer.

To answer your question-

You need to scuff it up a bit with scotchbrite, or something similar, followed by an Acid etch bath and then alodine the part. (The alodine is a chemical conversion process, which allows the paint to stick, turns it a bit of a gold color.) At his point the part needs to be cleaned with the proper solvent to remove any oils or other impurites from handling, if this is followed by a quality epoxy (two part) primer, and epoxy paint. It will hold up really well.

A middle of the road solution, would be to scuff it up well, and clean it well with solvent, and put a light coat of quality epoxy primer, something like DP40 or DP70. The put one or two light to medium coats of a good epoxy paint. The reason I say light, is even though this will make a tough coating, it will be subject to rock strikes and thus chipping. If the paint is thick it will often chip easier, and be harder to sand and paint over in such a way to hide the chips.