I would suggest a good oil base enamel since it is already primed. I used some paint from tractor supply on my trailer and am happy with the results. Unless you are building a show truck that will not have mirrors under it for people to gaze with admiration at your undercarriage I would not worry about it too much as anything you put there will look like crap after the first mud puddle, rain storm or snow fall. As long as it protects then you have done all you can.
