Line-X, Rhino Line or other
But if I only put it on for longer trips where extra fuel would be needed, I could probably live with the possibility of it getting hammered. Plus it's just sitting in storage now anyway.....Or maybe buy an extra tailgate just for this purpose. I see 'em on ebay and craigslist occasionally for under $100....hmmmmm......
If you get a spare tail gate, you might consider having the back side coated with a spray on truck liner to protect it from gravel, etc. while traveling dirt roads. I don't think I would do that to my only tailgate in case I wanted to sell the truck later, but heck, a beat up spare that needs a coat of paint anyway, why not?