If you take a look at
this video you can see Adam about in the middle (1:45, actually) kind of showing how a mold of a chest piece looks.
I'm really striving to make my pep armor great, because I'd like to mold and cast most if not all of the pieces. It's doable, but you're right, you'll want to seal it pretty much all up. I plan on fiberglassing the hell out of it for strength, and resining and bondoing the outside for smoothness and detail.