I painted mine 6-7 years ago and it turned out nice. I used Por-15 on the rusty and high wear areas, ramps etc. Lightly sanded the rest. Then primed and painted with impliment paint from Farm and fleet. It was a two part paint with hardner. As mentioned it is cheap, looks good, enough colors to keep me happy, and can be applied almost any way you want. I did spray mine. It has a nice durable glossy finish. That being said 6-7 years later it needs to be redone this summer as this trailer sees a lot of gravel roads.
With that in mind here are a few changes I will make. I will be heavily undercoating the inside of the fenders, they actually started rusting through from the inside in a few small places so they will be replaced and protected. There are undercoatings that harden but still go on thick. I for one hate the old style undercoating that always stays kind of sticky/messy so this hardening stuff is the way I am going. I am also considering the bed liner idea especially on the front a-frame where it angles in to meet the ball mount and the leading edge. This area takes a beating from gravel roads.