RE: Why Fiberglass?
Not only that but did you ever see a balsa plane that just had paint applied over the wood? Over time, due to the wood absorbing moisture from the air and then drying and repeating that for many cycles, the wood tends to check or split. The fiberglass tends to bind the grain of the balsa together and prevents that. It gives it a really tough surface and if done correctly, donsn't add much more weight than plastic covering. Monokote, if I'm not mistaken, weighs about 1.8 oz./sq.yd. If you use .6 or .75 oz. fiberglass, you can add over an oz. of resin per sq. yd and still come out the same. But you won't add that much because once you lay down the resin, you're going to squeegy off about a 1/4 to 1/3 or so of what you put on. Add the primer which is just to fill the grain, most is sanded off and then shoot the paint on and you will probably come out just a bit over the plastic covering and have a much more durable plane to boot. That's why we fiberglass.