I tried it on my front bumper once. I made an incredibly stupid mistake but it sounded like a good idea to me at the time. I decided I would want to chrome polish the bumper first so it looked good thru the paint, I proceeded to do so then didnt get a paint prep cleaner and left the polish on. Needless to say it didnt stick, Bubbled up green, And took me 4 hours to get it all back off. So my advice would be to just clean the bumper with there recommended paint prep crap. Don't polish it.
And i've been a little discouraged since that happened so I havent tried again with my new lesson learned.