I use light weight spakle... LePage Poly Instafill to be exact. I lather it on with a putty knife, let it dry, then sand it smooth. It makes for excellent wing tips and tail leading edges, and adds practically no weight.

It also takes paint well, and is amazingly durable. Although this may not be so applicable for open planes which often do not get painted.

For open planes, I sometimes use very thin MacTac - or sign vinyl.