I've just been seeing the updated build photos of Gilcrest's current batch of instruments [ It's so each owner can see the build in process] http://www.gilchristmandolins.com/current/
I saw what he was using to fill the F holes of his instruments prior to applying the finish and thought it was a great idea, sure beats stuffing newspaper into a instrument, as the balloon would be so much easier to remove.
Thought I'd share the finding.