wy metod :first I keep toghether the balsa sheets with adesive tape, then I apply thin cyano on all junctions, once the glue is dry I remove the tape and sand both sides wih a sanding block made of foam and rough sand paper, then I keep toghether the two skins(upper and lower) with many pieces of tape on trailing edge( to form it) I spread a little water on the balsa ,poliuretanic glue, place the core in the middle ......a lot of weight and after about 5 hours the balsa is perfetcly bond to the foam. if you cut the wings with built in dyedral you can also join the cores before sheeting and include the fiberglass between foam and balsa.