RE: Fiberglassing F20?
Although it may seem weird... I have seen GOOD glass guys prepare glass/paint pattern aircraft wings that weighed nearly the same as monokoted wings. Monokote is actually fairly heavy.
Sames went for some QM40 and Q-500 aircraft. Nice finish paint job, and no real weight penalty.
Use 3/4 oz cloth, Z-poxy or good finishing resin. Modern 2 part automotive finishes cover well and are fairly light.
As you noted, a little care and lots of sanding while building it will help.
I'd imagine with some care you could glass the F-20 with at worst a little more than a few ounces of added weight.