I don't think I've ever seen anyone put plastic covering on a fiberglass or plastic cowl, though I'm sure someone has done it. I've always seen them painted. The comments above about paint are right one.

Btw, the problem ingredient in the fuel is usually the nitro. There are lots of decals and paints that are fine with 5-10% fuel, but disolve when exposed to 20%.

I've built cowls out of wood and covered them with plastic covering with good results, but it does take a lot of messing around.