I just put mine inside where it was dry, although flash rust can be an issue. If you prime it, then you will have to (or should) sand it before you paint it if you want good adhesion. my bet is they will hit it with a rough abrasive, mine felt like sand paper. (if you are lucky, they will hit it with silica or something fine.....I had the pleasure of being the sandblast guy in high school.....I was usually more concerned with just getting it done than how smooth the finish was.) if you are happy with how smooth it is, get on! just sealing it up with a sealer and hitting it right after that with a top coat.
Are you planning on matching the boat paint job or going with something else? If you are wanting it to look really pretty, you will more than likely need to do the prep work I outlined. I put a base clear urethane on mine to match the boat stripe. I wanted to just use a single stage paint (same color, but eliminates the clear.....saves time and material). I did the base clear on the boat because it had two colors, since the trailer was one color, the single stage would have worked fine. Natuarlly they didn't make the that color in a single stage, so I had to spend the extra time and money on the base clear. it more or less doubles your painting time.
If you are wanting it just black or white or something basic, just use a automotive acrylic enamel over a sealer. Again how smooth the finish is will determine how glossy it is. You can also just hit it with rustoleumn, but the life span will be much shorter.
