Painted a fiberglass tube trailer last year with Benjamin Moore primer and Benjamin Moore gloss exterior house paint. Rolled on the primer and sprayed the top coat. I think the top coat would have done just as well or better if I had rolled it on. Regardless, the job came out looking very good. Both the paint and primer leveled well and did not leave roller marks. The paint dried very hard and has been reasonably scratch resistant. Washes well and seems stain resistant. Bonus is that it is water based. I would definitely do it again. Oh, and the reason I mentioned Benjamin Moore paint by name is that, in my hands at least, it is easier to apply and covers better than any paint I've used on the several houses I have painted inside and out. My (hopefully) last house painting job used 37 gallons of Ben Moore primer and paint!

One poster mentioned using a marine one-part polyurethane. That would be a great way to go. Interlux, Petit, Total Boat are all similar products that you can get from Jamestown Distributors. They have some good videos on how to use those paints as well. Those are made to be rolled on and level well. The only downsides to the boat paints are that they are twice the cost of the house paint and you have to clean up with solvent.