Tim,

I wouldn't use spackle to fill the core. If it is plasticized, in time the sheeting will delaminate. If the spackle/sheeting doesn't delaminate, the spackle/foam will come loose. I can't see any reason why spackle would want to adhere to foam long term. You are better off sanding the cores until they are smooth. If you wind up with an airfoil which is 0.5% thinner so be it. Just make sure your two cores are sanded evenly.

Something I haven't tried yet but intend to is to use some milled fibers in my laminating epoxy on the next wing. This might increase the weight of the mixture slightly (perhaps not, don't know which is heavier; glass or epoxy - probably epoxy) but it should produce an even nicer and stronger bond to the foam. Epoxy becomes brittle in time and glass mixed into it should help to keep it bonded - just a theory though.

Don't forget to make a slightly wider/deeper channel for the retract strut, you don't want it to bind against the wing should it bend slightly on takeoff. I would also open up the retract body well so it can be sheeted with some 1/16" balsa. Makes for a nice & clean body well which is easier to finish (I dislike spraying paint directly on to foam).

David.