When glueing the wing halves together, what you should be looking for is maximum surface contact between the root ribs. I know this suggestion will elicit many flamers, but epoxy isn't needed on the root rib joints. Aliphatic wood glue, or white glue, either one, will make a strong joint, and is both easier to work with and clean up than epoxy. The aliphatic and white glues will "wick" into the root ribs, and create a bond which will survive structural failure of the wood, be it balsa or ply. Clean the root ribs with the denatured alcohol, already suggested, and allow to dry. The key to a good joint is ensuring the root ribs are in full contact with each other. For the skeptics, I didn't know epoxy was supposed to be used for anything other than firewalls for the first 10 to 15 years I was in this hobby/sport. Never had a wing failure due to adhesive failure. Had several spars snap before I started using celastic on the center joint, though. I always did post-mortems on my piles of toothpicks, and none of my glue joints ever failed. The wood was always the part that failed.

Sure wish they still manufactured and sold celastic!!!!!!!!!!!!!!!