I hit the inside of our bumpers with Rust-oleum Universal Silver Metallic. It repels water really well and doesn't look that different from the natural color of the inside of the bumpers.
BTW, It kind of has a greasy feel so you can spray it directly over bolt threads - nuts will still thread right on. Also if you spray it on thick and you get runs, they pretty much level out and disappear.
Oh, and spray it outside away from anything you want to keep clean. This paint throws a LOT of stray metal flake into the air that settles on pretty much everything in your spray vicinity.
Hope this helps.