Same skarf joint as top skins. I used #100 sand paper in my orbital sander. But I bet #150 or even #200 would be OK. I sanded the outside of the inboard skin and the underside of the outboard skin.
Next I edge rolled the inboard edge of the outboard skin using my Cleavland rolling tool. I sure like this tool better then the one from Avery.
Time to dimple all the #40 holes. You must do the skarf joint and edge roll before you dimple. Can't do either if you dimple first.
