Door Skins

sure you can use fiberglass, just use 80 grit to scuff up the plastic and wipe it with a cleaner like acetone before applying the fiberglass. remember that the 'glass will not be as flexable as the plastic. An alternative would be a ueathane product that has a little flex in it.

Another option for repairing the panel would be to go to your local auto body repair store, like FinishMaster, and purchase one of their dual mix cartridge kits for this type of repair. The kits are available in rigid, semi flexible, and flexible materials with dry speeds ranging from one minute up to 30 minutes depending on your skill level.
These dual mix kits are offered by "SEM", "AUTOMIX", "3M", SURE MIX", and a few others. The directions for prep and use are usually on the product label itself.
I have used these products many times over the years and have had excellent results.
Good luck in your endeaver.