Yeah. sand the inside of the helmet first, give it a good cleaning, and then fiberglass outside in the shade. The fiberglass resin sold by 3M, is pretty ****** though. Especially when it comes to measuring out the catalyst. I think it calls for 12drops of catalyst to every 30ml of resin. But the tube the catalyst comes in, doesn't have a hole, so your drops aren't going to be consistent with their measurements anyways. Go to a decent marine supply store and get some polyester resin with wax. The wax additive, creates a barrier that seals the air out and helps the resin to cure. You won't need to go crazy glassing it. You can shred up some fiberglass mat, but it's important to make as even of a layer as possible. The thicker areas will get much hotter than the thinner, and could cause warping.