Wing/fuselage gaps: I have a balsa stripper that will make strips as small as 1/32 x1/16. These strips can be used to shim gaps in joints anywhere on the model and produce a stronger, lighter, and less messy result than fillers. If you use soft balsa stock, it will easily sand flush without marring the adjoining wing and fuselage surfaces. Also, for very small gaps, you can use Elmer's Wood filler, quickly wiping off the excess with a water-saturated cloth. When it dries, lightly sand it with 400 grit paper and carefully seal it with a bead of thin CA.