Myles

Is your wing capped with wood at the cord? If not, don't use bondo to fill the gap as it will eat the bare foam (Bondo is polyester based). Use epoxy and micro baloons.

I would fill the gaps and then glass it. That way you can overlap the glass cloth onto the fuse a little bit.

Next time you build, follow TommyWatson's advice and glass & prime the wings before you glue them to the fuse.