I've used light weight spackle on foam and solid balsa wings. Its the one that feels empty when that container is picked up. Each one I've tried had slightly different characteristics, some remained soft, some didn't sand well, and some worked great. The current one I'm using is Lepage Polyfilla Big Hole Repair. It sticks and seem to sand well and there have been others I've liked but some I've hated too.
Richard