are you sure you are using the right sand paper than? Wet or Dry paper in a 2000+ grit with water while you sand shouldnt be getting the plastic with 15 coats of enamel on it. What you would be seeing would be the scratches into the paint itself. Pick you up some Wetordry 2000+ put some water on the helmet, and try going to town with that, and I bet you'll see some good results.