If you must thin epoxy what works best is 99% Isopropyl Alcohol and then should be limited to 10%. Using an epoxy glue in any shape or form for wetting out glass cloth is just the wrong tool for the job. For glass work you really want a laminating epoxy resin. Z Poxy is most likely the easiest to obtain and can be bought in small quantities. Polyester resin could also be used but is heavier, weaker, more toxic and iron on covering does not like to stick to it. The term " finishing resin " is pretty much a farce, it is usually laminating resin renamed for marketing reasons.