So I bought this grille on ebay for my Charger. Of course it had cracks, weathered paint, broken off pieces etc.
First I've started filling in missing pieces. I've made a mold out of foil and put it behind grille and filled in with fiberglass. Once it was dry I've used polyester putty to fill in imperfections. Polyester putty works great on top of fiberglass and most plastics as it's more flexible than regular body filler. It's also recommended for plastic bumpers if you have a little nick or small spot that needs repaired.