[W]hich is this mylar technique?
I'll jump in here since it's been a few days. I hope Bob doesn't mind having his thread hijacked for a minute.
If I understand the question correctly, the "Mylar technique" you're interested in is a finishing method for wings and tails in which the base coat of paint is applied to a sheet of .014" Mylar, and then the Mylar is pressed against the balsa with epoxy resin and light fiberglass cloth in between. If the balsa skin is smooth, and if uniform pressure is applied to the Mylar sheet (as with a vacuum bag system), the finished wing or tail panel comes out looking as smooth as a porcelain dish. You can then apply your trim colors and a clearcoat if you want. Think of it as the poor man's billet mold.
Tricks and traps I've discovered:
1. Klass Kote paint is the best paint. It's a 2-part epoxy that sprays easily, cures very hard, and can be painted over with urethane, enamel, or pretty much anything else.
2. Use PVA as a release agent on the Mylar. You can wax it first with Johnson's Wax, but don't rely on wax alone. The solvents in the Klass Kote will compromise the wax barrier. Spray the PVA on, let it dry, spray the paint on, let it dry, GENTLY sand off any dust particles or bugs with #400 sandpaper, then apply wet epoxy resin and 3/4 oz. glasscloth over that using a squeegee or an old credit card and slap it on the balsa wing panel before it starts to cure.
3. Prime the balsa with nitrate dope if you have some, and sand lightly using #320 sandpaper on a sponge block. This reduces the amount of epoxy resin that will soak into the grain of the wood. My wings always came out heavy until I started doing this.
4. Also prime the balsa with a very thin coat of wet resin just before applying the Mylar/paint/glasscloth to it. I know it sounds like a contradiction of step 3 above, but the last thing you want is little "voids" of unsaturated glasscloth between the balsa and the paint. Most of the excess resin will squeeze out around the edges when you press the parts together. Obviously a vacuum bag will give more squeeze, therefore press out more of the resin, but you should be able to get acceptable results by setting the wing panel in the foam "shucks" it came from and weighting it down with paving stones or a small car or whatever.
Those are the basics. Someone could probably write a book on the topic, and maybe they have. But I hope this at least answers your question.