On bonding skins …

Make a “wiper” wider than the caps/spar by a little. But it
should be nearly parallel to the cap/spar at the edges and maybe 3/16” or
less high in the middle. If you’re not confy with the middle make it
taller. Since this runs across the cap and nothing is yet touching the area of
the wing skin that will be laying on the epoxy (structural adhesive) on the
cap, you can take and rough up that area a little and squeegee just a little
bit of structural adhesive over/onto it. Now apply additional epoxy and wipe it
with the adhesive wiper. Remove excess.  

Then put the two pieces together. Weight the skin down with plenty
weight.

After cure, you can look inside the wing with a mirror or scope and see
exactly how much squeeze out you have and sleep at night. This method does make
a nice job – perfect squeeze out for me was a bead of epoxy that was a
little smaller than a lead pencil all around the joint.

I made my wiper out of spare 1/16” plastic. Or you can make it
out of a 1/16” glass layup or what ever makes you happy.

Jim