Last year I did a combination fiberglass/plywood tilt-up camper for my Jeep-tub trailer. I used Olympic Rescue-It, which is a acrylic product for resurfacing worn decks and concrete. It's available at Lowes and other places like that, and they can tint it to pretty much any color (speaking of which, I picked up a gallon of it on the mis-tint table there for less than $20). It applies with a roller, and depending on the nap of the roller you use, it's got a texture somewhat like bedliner.
I did a test with a fiberglass scrap coated with it, left it out since October, through this worst winter ever, and today it looks just like the day I put it out there.
It's intended as a coating for wood, and since my tilt-up is a combination of fiberglass and wood, I used a quality primer on the whole thing before applying the RescueIt to ensure adhesion to the non-wood parts. The fiberglass sample in the photo above was primed except for the left end; on the left end when the finish was a few days old I was able to scrape off a bit of the finish on the unprimed side, but the primed side adhered perfectly. So if you're covering something besides wood, I'd recommend a primer; for wood you don't need a primer because the product is designed to be used on old house decks.
It's a very affordable alternative to bedliner for something like this.
http://www.olympic.com/products/olympic-rescue-it-wood-and-concrete-resurfacer