If the bumper is chrome, I think you could probably just scuff it and paint, but it may be hard to scratch properly, you may have to remove the chrome for it to adhere properly. A good coat of primer should also be used on any bare metal to bond well and give good corrosion resistance. If it is a plastic piece then you could use a fine scratch pad, some rubbing compound and adhesion promoter. And in either case, base clear paint will always look best.
