Tape can also work nicely when cutting the FG cloth. Apply the tape where you want, and cut down the centerline of the tape, leaving both edges pretty clean and tidy.
A very useful trick is to start by first lay a single layer of masking tape or electrical tape where you want the FG to stop (high point of the wing, or just outboard of the wing/fuse fillets, for example). Once the epoxy is hard, sand the FG down adjacent to the tape, until you get a nice trimmed edge at the tape edge. Better than having a few loose strands of glass glued to that soft balsa, where you'll get sanding divots.
I lay the FG on the surface dry, pull it snug, then flow on the epoxy. I think I've used the Z-poxy, but have also used various 30 minute types thinned with alcohol (shellac thinner). Blot up the excess with a roll of TP, disposing of the TP as it gets soaked. Don't f'get to clean off the surface wax with a cleaning solvent (Naptha, etc.). If you sand FG cloth too aggressively, you'll cut the strands, making for a hard surface...but without the strength you want.
Steve