Yes, epoxy. Glass cloth provides it's strength when it's locked in solidly. The epoxy does that. And it bonds it to the foam as well. Although, the bond to the foam is only as strong as the foam. So..............

If you're going to skin the wings, epoxy makes a good job of that. And to simplify the entire process, lay the tape with a thin application of epoxy, the same batch you're about to glue the skin on with.

I've built more than one (many more than one) big balsa-skinned-foam glider wing. A couple had no spar at all. Just skinned foam. It is amazing how much strength the skin imparts when it's got a chance to soak in some epoxy from the attachment process. And for most glider wings, I would be perfectly happy with a sheeted foam wing with just the spar for added strength. If your sheeting is sufficiently thick, and decent balsa, you'd be amazed at just how strong it is by itself. The camber provides a great deal of that strength. The foam is just strong enough to provide support to keep the two skins from buckling inward. Add any kind of spar and it's really good. And think about it..... you're going to stick the spar wood in the foam sandwich with what? epoxy..... which then does for the wood fibers what it does for fiberglass cloth.

Way back in the day that foam was a new method, we'd cut the blank from root to tip on a line where the spar would go. We then cut some balsa sheet into many pieces and run them out the wing. We wanted the grain going up and down, not spanwise. We'd stick the pieces between the foam pieces and press them so all the excess was sticking out the top surface. When the glue was dry it was simple to use a sanding block to bring that one "fence line" down to the foam. Then we painted the inside of the skins and etc etc etc.... BTW, paint the wood with the epoxy, not the foam. And once the wood is coated, use a playing card to scrape the epoxy to uniformity. The time it takes gives the epoxy just the right amount of time to penetrate the wood. And you'll have more than enough epoxy to stick the wood to the foam.

The toughest aspect of any epoxying job is putting on no more epoxy than needed. We almost always wind up with too much.

Good luck on the wing no matter what you do. Post some pictures later.