It works fine for filling in fiberglassed models but not sure how it works on balsa when applying heated covering to it. It's very similar to the acryl spot putties used in the auto repair industry but a bit less runny. Plastic modelers use it all the time to fill voids in their assemblies. It sands fairly easily, dries quickly and you can scribe lines in it. I've used auto body spot putty, or glazing as it's sometimes called, to fill small voids in planked fuselages and that worked fine.

The thing that most people make the mistake of doing when sanding is they use too fine a paper so they end up polishing the hard spots and sanding down the softer spots. If you use a fairly rough grit like 60 to sand the putty down, you'll find that it comes off just fine and when all is to approximately the state you want it, than hit the whole thing with 220 to get rid of scratch marks. You'll be glad you did.