100 grit for finishing??? On Balsa? I think Id sand right through my wing sheeting with that. I don't use anything less than 150 grit for finishing, I save 60-100 for block shaping use (along with a hobby knife and plane).

What I do is I first use lightweight filler (model magic, etc) to fill in any dings, scratches, etc. Then I sand the whole surface down with 150 grit on the largest block that works. Now check for any low spots and reapply filler as needed. Sand again with 150 (lightly!!!) and repeat until there are no low spots.

Be very careful with this first rough sanding. If you have thin wing sheeting (like 1/16 balsa) even 150 can sand right through it quickly.

After this I work my way down through 220, 320, and 400 grit, again using sanding blocks. A couple minutes with each grit is all that you need on balsa. Note that to get the best finish, sand with the grain of the wood here.

Once you are satisfied with the finish, prep the surface for covering by vacuuming or wiping with a tack rag (or both).

Finally, take you model out of the shop to cover it. This way all that sanding dust cant get under the covering!

Good luck,
Jeremy