If you've got the money, Line-X. If you don't have the money, I've used stuff calles Hippo Liner (not rhino liner), You can get it at autobody supply stores, and it comes in two cans that you mix together, then you can either roll it on with the supplied roller, or spray it on if you have a gun with a 3.0 mm tip. It's really good stuff, very thick and durable, and the closest thing I have found to Line-x. The best part is it's only about $60.