roll on or spray on truck bed liner is definitely the best possible coating for this application. Not only is it extremely water proof, but it is nearly indestructable, so your kegs won't chip it no matter how rough you are. The trick with this stuff is to make sure the surface is extremely clean. After thorough cleaning whipe it down with a cloth with a tiny bit of mineral spirits to ensure a strong bond. Watch out for the fumes, they are REALLY nasty.