You can do so, but you would going a more expensive route.
You don't have to use Fiberglass. Its just another optional step to strengthening the PEP work, without adding weight.
Most people are fine with just Resin and rondoing a couple of layers, and then using putty to fix any imperfections. but of course, Bondo/Rondo is a lot heavier than using fiberglass.
What is it about fiberglass that you are worried about?