I have been demonstrating the model I did this way, but seem to be having a "problem of communication" .
The plan is to attach the Polyspan to your model by your preferred method (dope down the edges or iron down over Balsarite or Stixit). Do NOT dope the open bays! Heat shrink it as needed.
Now, cover the surfaces with SLC or perhaps Monokote, and then, using your iron, bond the film to the fiber. You are done,unless you want to paint it.
You now have a slick, smooth, remarkably tough surface. Sand the areas where you want to paint with 400 grit, wet. No primer needed, you have a flawless surface base. Shoot your base color and go from there.
I would expect to save several ounces and a whole heap of work on a full size stunter. Let us know your experiences with this technique!
Due to engine problems, my model has hit inverted a couple of times, and the material will get abraided, but the structure was undamaged, nor did the damaged areas " spread". A bit of clear tape, and back in business.