I would polish or light cut compound to start and see what that does....I have had decent results with this process. I have also never wet sanded the bumpers....They are clear coated but I would be afraid that you could go threw that quickly and hit the paint or below that......if this was a customer of mine, I would tell the customer that taking these out can be tricky and you will give a best effort. On my Jeep I slide a box across the bumper and it rubbed right threw to the plastic pretty quickly that was enough for me to think that wet sanding might not be a good idea.