If it's anything more than can be sorted with Halfords Bumper Black, may I suggest you call in the Chipsaway people?
Normal aerosol paints don't stick to the plastic, and special filler and primers are needed.
John
The primer and paint need to have plasticisor (sp) in which allows the paint to flex with the plastic bumper - if you use standard spray paint it'll crack and peel with the slightest flex