^^ what they said Resin will help plug some little holes in the pep if there are any too so the rondo doesn't spill everywhere. learned that with my first piece i ever did. make sure to add some support struts too if there arent any. it will help keep the parts shape.
I prefer fiberglassing the piece before Rondoing. This to help strengthen the piece, and keep the rondo from warping your piece. I Rondoed an ODST thigh once and it warped like hell. I would not recommend Rondo as a way to substitue fiberglass. Only thing Rondo is good at is to get to those little nooks and crannies that you wouldn't get to otherwise. (Like the brim of a Helmet.)