RE: Fiberglassing questions: Why Bondo Specifically and Regarding Seams and now, more
MY opinion on thinning epoxy....
If you are only using the thinned epoxy to apply light weight cloth (3/4 oz or lighter) the thinning agent will evaporate fast enough so the strength is not affected enough to matter. You are only "gluing" the cloth to the surface not using it as a structural component. If you are doing a layup of a fuselage or cowl, never thin the resin as the strength will be affected. The thinner has no chance to evaporate from the thick layup.
Have you looked at using the water based polyurethane varnishes to glass with? Never done it myself so I can't really comment. I'm told it is a bit lighter than epoxy, not quite as durable and less messy as it is water cleanup. I guess you need to watch the product soaking into the wood as it is water based? Some are using some sort of sealers as a first coat.
Ken