OK, can help here too, I've done autobody for more then 25 years (yes, have a business in this too :lol ). We've built everything from wings to body kits to entire race cars.
If I understand and have followed corectly, right now you have a "Chipboard" surface? I'm guessing 14 ply at 0.05 - 0.10 inch?
You have a few options you could choose from, all being strong, long lasting but some work involved. I'll list easy to hardest here for you:
1. "Filler Primer" (Dupli-color comes in a rattle can) will be paintable and sandable as well as filling in any small gaps, pin holes or such. You can also get it in un-mixed Quart to Gal. size at auto paint stores. Getting the larger amounts will then mean you'll need a compresor and a gun to shoot it with. So that's a bit of an investment. Of course you could check and see if there is a Rental place near you that has compresors, then you could just get a simple gravity feed gun for about $25-40.
Bad side: this won't stiffen the body, may flex and crack and you'll have to seal it inside and outside. You'll also need to lay the paint on rather thick.
2. "fiberglass resin" This'll be like what the guys who do those paper props do. A ~skim coat~, meaning very thin, cover the entire surface. This'll be tricky as you can't mix enough to to the entire piece in one shot, you'll have to mix as you go. It's kind of like painting, apply wet to wet. So as you do a section, the next batch extends, overlapping, on the one you just did. For the size piece you have there, I'd have 10-12 "cups" ready to mix up. Allow it to set up hard for a few hours, then sand the entire surface LIGHTLY with 400. Then apply another coat of resin.
Once the second coat is set hard, sand that, look for low-high spots and fill-sand as needed. You'll find pin holes and need to use some glazing puttybefore the final sand.
3. "Bondo" I don't recomend this, it'll take a long time to sand it to shape, be messy as heck (not to metion the dreaded ~bondo boogers~! :lol )...But it'll be the cheapest way to go. Bondo is a lot cheaper en-mass then resin. It's also not as forgiving as resin and can crack when flexed.
Hope this helps and if you need anything else let me know.
GREAT job so far, really well done.
