I hate it when that happens! In the future use Pica sandable glue. Drys fast, strong, and sands more easily. The problem is that all white glues are harder than the balsa, so when you sand over it you sand more balsa around it than the glue. The only thing you can do is carve out some of the glue joint and patch it with your favorite patching material. Is this the joint between the wing tip and wing? Or did you have to splice shorter pieces together to make you giant wing? If you use fabric covering it will hide the joint somewhat.