I agonized over making this scarf joint where the inboard and outboard wing skins over lap. In the end it was no big deal at all. All though I haven't seen the two skins on the wing yet, but I think it will turn out OK.
I know the drawings show that you should sand away a trangle pattern, but this just doesn't make sence to be. My sanding area on the inboard skin was the size of the overlap inboard/outboard and 3.5" leading/trailing and was made on the outside.. The sanding area on the outboard skin was a bit bigger and was made on the inside of the skin.
Image 1 shows some practice joints. I used 100 grip sand paper in my orbital sander. Place the sander in the area you want to scarf, then lean it, with a very small angle, towards the corner to get the taper. Keep the angle small. If you don't you'll be sorry.
