The quality C/F tubs have a liner epoxied to the bottom side to protect them, but it typically covers only the center area over the tire. IE, doesn't cover entirely above the tire. I have heard of people making a thin pc of alum sheetmetal to make a rock guard as wide as the tub and attaching it to the underside up against the c/f tub. I would thing a pc of .032, a few rivets and some blk paint would hide it nicely and do a world of good for a street car.