Can anyone tell me why sheeted balsa, electric aircraft can't be covered with water base polyurethane varnish instead of using a Monocote type product? We obviously don't need fuel-proof finishing. I have no idea what the weight penalty might. Thanks for your input.

pguiet, Most of the guys that use fiberglass mesh over foam wings use polyurethane to attach/fill
the mesh instead of fiberglass resin. I don't think the balsa/poly would have as much strength as the iron on covering or fiberglass mat and poly. I don't know if this helps or not.
Happy flying,
Bud

You can cover it as you suggested. I cover all my fully sheeted planes with Deft....a laquer, then paint over it. Lighter than any plastic film if you don't put on too many coats of paint.

Minwax polycrylic is what your looking for. Chech out similiar post under "foamies--Finishing Foam".
Happy flying,
Bud