Sean,
Another product that I've been wondering how it would work for your application is a product that I use all the time on cars.
It is by Evercoat and is called "Slick Sand". It is a high build Polyester primer. It sands awesome, you can apply paint directly to it, it fills fantastically, can be reduced to flow out smooth, dries quick, and once it's cured it is solid. That might be it's only flaw on a thin lay up like a fuselage, although it works fine on urethane and fiberglass bumpers.

https://www.autobodytoolmart.com/p-1...ID=41632055522

It's cheaper than the K36 as well. For cars it's awesome, I can spray on 3 or 4 coats and then block it and it's build is thick enough to get rid of 80 grit scratches as well as small dings. Of course I wouldn't use it that way on a jet or any plane, but the build is awesome, might be worth trying.