The first task for today was to use my micro-stop drilling jig to machine countersink all of the holes on both sides of the trailing edge extrusion. This will allow the dimpled skins to nest into this extrusion for a flush surface on the trailing edge of the rudder. After that, I started preparing to back-rivet the ribs onto both halves of the rudder skins. After loading up the rudder skins with flush rivets, I taped them in place with some blue painters tape and then flipped the skins over onto my bucking plate. The ribs could then be easily positioned onto the tail ends of the rivets and back-riveted quite easily. The one trick that I did this time was to use a small refrigerator magnet to monitor where the edge of my bucking plate was, to avoid accidentally riveting beyond the edge of the plate (which would easily dent/destroy the skin). What's next? Time to start assembling the skin halves together and putting them onto the rudder skeleton.