Firstly, never, ever use WD40 for this job. Reason being, it is a water attractor and works as a water displacement fluid by virtue of the water falling away with the excess WD40. It's also a rubbish lubricant as well.
If the inside of the bumper is not plated, my temptation would be to use a phosphoric acid rust treatment (Kurust is one) and then paint it black, or body colour.
