This will require a DA or some sort of orbital buffer but polishing compound should work, WITH a DA. As an experiment one time, I stripped the paint off a broken 320i wheel and got it to a bright polished shine with rubbing and polishing compound. Don't even think about it without some sort of a power tool as your arm will fall off first. Try the bottom of the bumper first if you're not sure, that way you aren't experimenting with what you will be looking at.