years ago there was a rubber type mat that had adhesive you just stuck on the underside of the fenders similar to the dynomat you use inside for noise & heat reduction, but I haven't seen it advertised in a long time. this would have been 20 plus years ago, way before bedliner spray was invented. the only other thing I can think of is rubberized undercoating, but I really think you wouldn't be happy with that. I wouldn't.