Sand down epoxy on wing top surfaces

The rivet filling epoxy was sliced off with a blade, nearly flush with the top surface of the rivet. However often a thin film of the epoxy was left. If the rivet head had any swarf (more common with the 3.2mm rivets where some material would be pulled into the rivet gun head hole), the blade was not sufficient. To remove this debris, improve the uniformity, and ensure a good key for primer, sanding with 240 grit paper was applied. Various thickness masks were used to prevent removal of more than the slightest amount of rivet metal.