As a one time thing you could spray it with mould release, would do nothing for the rocks that get kicked up but it washes of with water and the wax job underneath would be in great shape. I have seen soft pillow like covers (custom I think). you have to have a cover that will not rub or flap in the wind, but will stop a rock, and is soft on the inside. sounds like a job for the off season and someone that can sew.