Check out automotive paint and body supply shops, they should have a primer option specifically for plastics. Bumpers and trim are all plastic nowadays and painted to match the car, prime it with the proper paint and then you can paint it with whatever you like with minimal fear of the primer failing and the new finish flaking. Most shops will be able to custom make spray cans of automotive paint and clear coats as well, a satin or flat clear is a good idea to finish off with.