Dear Friends,
I try to answer in logical order:
Why this method?
Very simple: in the early '80s my conventional balsa built-up models suffered from strong resonance. I tried to detune them from the engine's useful RPM interval, but no success. Finally the long pushrod broke, killing the plane & engine.
That time the white foam+balsa (or Obechi, etc.) planking method was known already, but in lack of vacuum-technology, and self-expanding, light polyurethane glues, I did not consider to be reliable and light enough. (And technically "elegant", as well
So I developed my method on extruded Polystyrene foam. As I observed under microscope, the blue has approx. half a big elementary bubbles, than the pink brand. All other parameters are similar. (Material, density, strength).
I find good finishing/laminating epoxy resin (nowadays is Huntsman (CIBA) LY 5052 type), and the steps are as follows:
1. Making the foam by usual methods: hot wire (not too hot, to avoid grooves), sanding by sand-canvas (120 or 150 grade), and/or trimming by balsa-plane, using razor-blade (!!).
2. Stabilize the surface by resin: I thin the epoxy resin by methanol, adding 20-30%, plus some talc, to better sanding ability. And "paint" the surface by this mixture. Do not worry: a GOOD resin will be hardened for tomorrow, anyway...
3. Sand the surface by sandpaper, grade 280. Even by wet method: the Polystyrene foam is NOT sensitive of water: how a big advantage over any wood!
4. Cover the surfaces by light glass (25-28 grams/sqm), or carbon tissue (9 grams/sqm), or light unidirectional carbon (where extreme strength is needed: in the central part of wing, horizontal stab.) The resin can be the same, as in point #2.
Inside parts (e.g. spar, etc.) must not impregnated by thinned epoxy.
5. Sand again, as written in #3. The surface is ready to paint. (I use to order nitro-based colors from auto-paint shop, covered by two-component acrylic hard lacquer.
Tomorrow I collect the measures and weights.
Regards: Istvan