ask for automotive paint stripper. its alot stronger than ordinary household paint stripper. apply it as instructed on the back of the pack. if you let the paint stripper settle in (and provided you dont have many layers of resprays upon resprays), you should be able to scrape it off real easily. this strips the paint only, and not any previous bog left on the bumper bar.
one strong word of warning, the stripping agent reacts with certain plastics! alot of modern bumper bars are made from cheap plastics (even if its a reputed japanese brand), and i've heard of numerous cases of the bumper bar melting. however, if you're doing an old bumper bar it should be fine. do it at your own risk though!
most panel shops prefer not to strip bumper bars. they will either spray over an old one, or have it replaced with a new bumper bar.