I resumed work on the upper wing skins by using an oscillating tool to remove some material from the overlapping seam corner to make the stackup thinner, as per the instructions. After measuring with my digital calipers, I think I will have to come back to this and remove some additional material.
Then i started scuffing the backside of this skin to prepare it for priming, and then dimpling it (except for the area needing additional material thickness removal). I also used the edge rolling tool to add a slight bend on the leading the overlapping edges so that they will sit flush when finally riveted.