I use Plast I Weld in a Touch n Flow Applicator for most of the build specially for joining fuselage halves, filling in thin gaps and getting rid of join lines. I run a bead on top of a seam then squeeze both halves creating a ridge of melted plastic. The part is clamped and set aside overnight to cure. Next day the ridge gets sanded to an invisible solid join. If a ghost line is still visible then I run another bead on top to melt the plastic and sand once its cured.
Narrow gaps like along the wing root are filled with Plast I Weld in a Touch N Flow applicator melting the plastic insise the gap and closing it.
I also use Tamiya extra thin and Revell liquid cement in the blue plastic bottle with long metal applicator tip for certain applications.