Here is my opinion:
Let's use the photo above as an example of a pep'd piece that you want to turn into fiberglass, and discuss. Teddz' piece looks as good as most peps I see. Some are a little better, some a little worse, but that looks to be what most people would come away with, so I will use this for our discussion.
many of the builds that I read, describe a process similar to: 1) cover paper in resin. 2) Fiberglass the inside for stiffening. 3) use Bondo to smooth out the outside.
I don't like this process, as though we were trying to sculpt in Bondo. This requires a LOT of sanding, re-filling, more sanding, more filling, more sanding.....Until done. THIS IS MESSY, ITAKES A LONG TIME, and You have only a 50% chance of finishing something worth displaying.
Let me make a suggestion (not just to you AJ, but to all future pep'rs): once you have the finished Pep, and you want to turn it into a fiberglass piece, try this:
1) spray with lacquer to seal and protect the pep from moisture. Give it a few extra coats to stiffen. This is much better than brushing resin onto a fragile piece.
2) Reinforce the inside with resin, and add popsicle sticks or other material, so that the bottom sits flat, the diagonals are even, and even add some spay foam to stiffen the model.
3) At this point, you should have a model. NOT a finished piece, but a model, and we will treat it as any other model, we mold it.
4) This is known as a "Waste Mold, because it only gets used once. Hyrocal is a gypsum that is cheap and readily available. Silicone RTV will also do the job for this, but is cost-prohibitive.
5) You must create a dividing line for the mold. plaster is heavy, and will warp a paper pep if it was not sealed or supported well. Apply a thin coat and let set, so as no not crush the pep. next, add an additional layer of plaster, and reinforce with burlap.
6) Once cured, open the mold and remove the pep. The pep is destroyed here. So if you didn't do well on the mold, ALL you effort is for naught!
7) peel out the remaining paper pep, and throw away. This is where is does NOT matter if there were undercuts, or other "locks" in the model. because the model is destroyed in the removal, NOT the mold. (this is also true of foam models, they get broken out of the waste mold!)
8) Now, you have a plaster mold of a pep model.
9) You CANNOT lay fiberglass into an untreated plaster mold. So let the plaster mold cure for 1-4 weeks. (Plaster hardens in 10 minutes, but takes a full 28 days to fully cure)
10) Once cured and dry, the mold can be sanded of any high spots (low spots on the model) and modeling clay can fill any low spots in the mold. (which would be high peaks on the model)
11) Spray the mold with several coats of sealer, (Lacquer) and lightly sand in between coats.
12) spray several coats of PVA or comparable mold release.
13) lay fiberglass into mold. (It is ALWAYS my preference to use 2 layers of 3/4 oz matt, with a final layer of 6oz cloth - because if it is going onto your head, smooth is better than prickly!!!!!!!!!!!!!
14) Once the fiberglass has cured, (and this is a HARD one) smash the mold and break out your new model.
That's right, you did ALL that work to smash it on the ground
Now, what is left is a fiberglass copy of your pep. It is not as pretty as you would like, but you can now do the Bondo work on a solid fiberglass base.
I will NOT tell you that grinding and sanding fiberglass is any fun, but you are likely to have more success than trying to Bondo a paper model.
My advice goes down hill from here. If I were to make this model, I would do an intermediary step. I would pour plaster into my plaster mold, and produce a plaster model. Why? Because I can clean up a plaster model a LOT easier than a fiberglass one. The plaster can be shaped with chisels and sanding, a little filler and primer to get a finished model. ...and then you need to make another (finish) mold to create a clean helmet.
Those are my thoughts on Pepakura models. There is NO simple method to turn a paper model into fiberglass. It takes a few steps. The point that I wanted to drive home, if you are still reading this far, is that trying to Bondo a paper model is very time consuming and not the best choice for good results.
feel free to discuss amongst yourselves.