Here's my take on this....

The theory behind using hairspray is to glue-down any balsa dust. Problem is, you're gluing it down with a water-soluable spray that is intended to hold hair down for about 6 hours (before humidity washes it away).

Wipe your wing with a tack cloth before covering and you won't need it. If you still feel the need to glue down the dust, use Balsarite, dope, or something else that's a little more permanent.