Is the car painted? If not there are quite a few products that you can use as a skim coat for that. Also filler primers work well but you have to have a flex agent in it for it not to crack on the flexable bumper. Do you have a paint supply shop in your area? Your best choice is to drive down there and talk to the guy he'll give you what you need and good advice.
Well if it's a flexable bumper cover you need to use the right material to fill it with. 3M makes a two part filler for fixing flexable bumpers with problems like that. A skim coat is just a very lite coat of filler dragged across the repair. Make sure you sand around the repair area, feather the paint apx. a 1" away from the place you are repairing. Always apply a aheasion promoter to the repaired area before you apply filler, primer or paint to a flexable bumper. This can be just be applied with a rag most times. I would spray it on when it comes to doing it just before painting. You don't want to do all that work and then have it peal or lift up on you. Ask me how I know this. Hope this helps Tim