This is from GP Shoestring manual for joining the wings together. It should work for your Piper Cub also. Mixing microballons to the epoxy makes it thicker so it stays in place and fills the gaps around the wing joiner. Use epoxy without the microballons for actually joining the wing together. Hope this helps.
❏3. Prepare 1/2 oz. of 30-minute epoxy. Add 1/2 oz. of
microballoons. Working quickly, thoroughly coat the inside of
both wing halves where the joiner fits and one half of the
joiner with the epoxy and microballoons mixture. Making
certain the joiner is upright, insert the coated end into one of
the wing halves. Coat the other end of the joiner with the
remainder of the epoxy and microballoons mixture. Proceed
immediately to the next step.
❏4. Prepare an additional 1/4 oz. of 30-minute epoxy only
(no microballoons). Coat the ribs on both ends of the wing
with the epoxy. Join the wing halves tightly, holding them
together. Use a paper towel to wipe away excess epoxy that
comes out of the wing. Tightly hold the wing together with
masking tape, making certain both halves are in full contact
and that the leading edges and trailing edges align. Wipe
away excess epoxy and do not disturb the wing until the epoxy
has fully hardened.
You can find the Shoestring manual at this site for more info on how to join your wing. Takes awhile to load so be patient .
http://www.greatplanes.com/parts/