Once you get all the parts edge-deburred, you assemble them for the first time. Here's the guts of the vertical stab:
Now that the spars and ribs are assembled, you put the skin on and match-drill. The parts all have this blue vinyl stuff that protects them during shipping. You pull that off. On the exterior surfaces I cut it just around the rivet holes to leave some protection from scratching. Some say this is overkill because it all gets scratched up as part of paint preparation anyway, but I still do it.