They now make a filler just for rubber (plastic) bumpers. I used Evercoat Poly-flex for my car.
When repairing a rubber bumper is is imperative to clean, clean, clean all traces of dirt, silicone, or other contaminants before doing anything.
I violated the rules of purists by painting my bumpers to match the car. I love the looks of it this way, your mileage may vary.
If you want to paint the bumper using real auto paint, just add a flex additive to the paint.