The rivet holes in the wing walk area of the top skin get counter sunk because it is two layers thick. The top skin counter sink results in close to a knife edge so you have to be careful with this operation. I used my variable speed electric drill and went slowly.
I trial fit the outer top wing skin panel- beginning to look like a wing!
The goal of the feathering is to have the skin edge match the thickness of the tank skin which will be butted next to it. Here is a scrap of aluminum the same thickness as the tank skin placed next to the scarf joint.