I have built a total of 47 kits and/or ARF's to date from .30 to 2.4ci and use this method...

1) Get some fiberglass cloth. Wing center section glass can be bought (about 4"-6" wide) but regular glass works too (7.5oz or heavier).
2) Remove the covering from the center of the wing being careful not to cut into the wood.
3) Join your wing halves as normal per your instructions.
4) Roll the fiberglass on a small dowl (or similiar) as it makes it easier to handle.
5) You can either use epoxy or CA (personal choice).
6) Start by tacking down the end of the fiberglass at your starting point.
7) Soak the cloth enough to get it stuck down well and if using epoxy use a credit card (or similiar) to remove the excess. If you are using CA, just work the CA into the cloth with a finger covered with a plastic bag. WARNING ! The fumes from the CA when done this way are powerful ! Watch out for your eyes ! I have come away looking like a had been crying for 30 minutes while doing this so be careful with CA !!
8) As you go, unroll the fiberglass on the dowl and work your way around the wing.
9) Trim the excess and stand back and admire your work ! Now get the CA off your finger where the plastic bag got a hole in it .
10) Recover the center section with matching covering material. I do some prep and fill the glass with some Model Magic Filler (I have used spakling for this too but its heavy) before covering to make it look seamless. I just apply the filler, let it dry, then knock off the excess and sand smooth. It may take two coats of filler to get a good smooth surface.

That is what I do and I have never had a wing fail on me. I am sure others have other ways that will work. I built kits in the early 80's (mostly Goldberg) and this is how the instructions say to do it for most of their kits. If you have any questions or I was not clear on something, just ask !

CJ