Foam core wing vs. built-up
depends on what kind of plane it is and what weight and span, but even for most jets a foam wing with 1/16 balsa skin and a layer of 1-2.5 oz cloth is fine. You can use epoxy to bond the skin, and under the skin surface, the side that gets bonded to the foam, run a strip of carbon tow on the CG, or MAC, this way you wont have a bump if you wet out the tow well and spread it out a little on the balsa.