First you will want to wipe the trailer down with a quailty wax and grease remover to get all the oil and other chemicals off. Then sand the trailer down with 180 or 220 grit sand paper. You could use a self etching primer but unless you have a lot of bare metal it is a waste of money. A primer of like that is used when something is bare metal to inhibit rust from forming it is expensive, and I do not reccomend it. There is another primer that fills in scratches so if you don't have a lot of scratches don't use it either it cost around 100 dollars a gallon. If you want to put something on the trailer before you paint it I would suggest putting on a primer-sealer. Then go ahead and paint the trailer. I would suggest you go to an automotive paint store and get a good automotive single stage paint. NAPA also sells good paint if you have nowhere else to go. A decent paint can be bought for around 100-150 dollars a gallon. You can buy better paint but I don't reccomend it for a trailer considering it can run almost 400 dollars a gallon. Hope this helps out.