If its metal underneath it is fine. I'm not sure why you are putting it under there, is it rusting? My parents had bed liner sprayed on my car to stop the rusting, from the molding down to the door sills is bed liner and then on the parts above the wheels is bed liner, it looks better than it sounds lol. and its been great, its rubber so things like shopping carts just bounce right off, the only thing is, if there's any plastic it will not stick. on the bottom of my car like where you step in is plastic and the person sprayed that too and it has peeled off.