I've got my front bumper done with Rhino-liner (professional bedliner), and the skidplate attached to it with flat black rust-oleum.
I've only been able to put one "scratch" on the bedliner because it's...well...bedliner. It's tough, and the even with all the rocks, grit, and chemical crap on the roads in the winter it doesn't seem to harm the bedliner at all.. Touch-ups are easy - wire wheel and rattle can.
The skid plate is a different deal - it scratches waay easier, and it's starting to rust a bit...but that's okay. Touch-ups on that are easy too.