[cycleguy55]

If it's inside wheel wells or on the back side of a painted surface, perhaps: a) Plasti Dip; b) undercoating; or c) truck bed liner. Plasti Dip is the thinnest, available in multiple colours (including clear) and it can be peeled off if you decide you don't like it. Undercoating and truck bed liner are much thicker, heavier and I think pretty much permanent once they've dried or set.