That 3M 77 Spray glue usually attacks the foam. Unless you put a very thin layer of the 3M 77 contact cement on the foam wing, it will dissolve the foam. In my experiences, the 3M 77 spray glue usually seperates after a while. Using too much will ruin the foam, using a thin layer usually doesn't adhere well.

Dave Brown does not make sorgum glue anymore. Sig makes a great contact cement, but it is very expensive. But in my opinion, is the best contact adhesive anywhere.

I find that Elmers Carpenters wood glue is great for attaching balsa to foam wings. It is just as strong as epoxy and less than half the weight.