Riveted skins to ribs up to stringers on top side and then bottom. Next riveted the remainder of the ribs, except for the tip and inboard ribs (which will be squeezed last). Fitted the rear spar into place and set the LP4-3 rivets and started to cleco the skins to the flange when I discovered that I had not countersunk the spar for the skins. Drilled out the LP4-3 rivets and removed the spar, then spent some time removing the back side of the rivets from the HS ribs. Decided to leave the counter-sinking and remainder of the riveting until the next session.