I hung the skins on the wing frame to debur the holes. Just easier this way. I do the underside this way also.
I made the scarf joint on the bottoms skins. I think I sand down the upper skin too much. I was looking at the drawing and it shows the skin underneath should have most of the beveling. But I tape off the place I want to sand down. I use my little electric sander and then I scotchbrite to finish off smooth.
I also flied the edge of the inboard skin that goes under the outboard skin so that there won't be a lip under the skin, but rather a smooth transition off the bottom skin. This is actually shown on the drawings.
I also rounded and smoothed the outboard skin's, inboard edge because it is exposed and I wanted it to be 'nice.'