Super Duty,
I used to do a lot of work on ocean boats, so rusty trailers is something I have seen a whole bunch...This will sound funny, but I would not paint the trailer but would rather powdercoat it with a zinc based primer and powdercoat it with the color of choice over that....Powdercoatings only real drawback is it is difficult to repair if it gets damaged, but is tougher than paint and is cheaper to do (paint, especially in California is stupid expensive)...
Depending on who does it in your area, it only takes a day to coat it.....unfortunately, you need to use metal containing fillers for imperfections.
My pops' Southwind trailer has been bedlinered and painted, it does not look bad at all and is tough as nails....Just a different look is all...
GT