Glassing wing center
Either will work fine. Epoxy resin should be a little more flexible, and that is what I use (20 minute finishing or regular 30 minute). Polyester finishing resin should work fine, though, if that is what you already have. Another option is to use thin CyA glue. Great Planes suggests that method in some of their instructions. I have used it to apply fiberglass to the inside of cowls at the screw points, but I've not tried it on a wing joint. Whatever you use, make sure to squeegee it out, so you don't get too much resin under the fiberglass. Keep it thin and tight against the balsa to get a strong bond and no bubbles. If you use CyA glue, use a piece of clear plastic baggie on the tip of your finger to spread the glue.
Pilgrim