Their is a plastic filler available specifically for bumper and or plastic repairs.Companies like 3-M,Evercoat,SEM,Fusor have plastic repair filler available.Most are really easy to use and are very flexible you must follow directions carefully.Some may require an adhesion promoter.Check with your local automotive paint supplier they can help you out.Also even though you may have some experience painting it may be cheaper and less headache if you took it to a reputable body shop.Cost of proper paint supplies and equipment may be higher.
Sam