They now make a filler just for rubber (plastic) bumpers. I bought 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. If you want to paint the bumper using real auto paint just add a flex additive to the paint.
I violated the rules of purists by painting my bumpers to match the car. I love the looks of it that way, your mileage may vary.