Time to rivet half of the rudder stiffeners to the inside of the rudder skin. With the rudder interior primed and ready, spent a few hours setting 100 rivets to get the stiffeners in place. Ironic that it's exactly 100 rivets. Only thousands and thousands more left to go! The first three I set were not great. Structurally ok but the shop heads were junk. As long as the tech counselor is ok with them, I will leave them, if not I will drill them out and redo them. After the first three ugly back rivets I remembered that I had a different back rivet set that worked much better. Quick change to that rivet set and the results were much much better for the remaining stiffeners. I only made one mistake when the skin slipped off of the back of the back riveting plate. My back riveting plate is only 6"x14" so not that large. Lesson learned. The mistake resulted in a small reverse dimple on the outside of the skin. After a quick check online, the consensus was to leave it be and build on. After the first few stiffeners I realized that the rivet set was scuffing the primer off of the stiffener as it was being used. So out came the masking tape, lots of it! The very last rivet on each stiffener is very close to the trailing edge of the rudder which meant having to really bend the opposite side skin back quite a bit to get the rivet gun in there. Doing so resulted in more scratches to the primed surface, grrrrrr! So more masking tape and by the time I had finished I had worked out which areas to tape off to prevent scratching to the primed surface with the rivet gun or rivet set. So lessons learned: 1. Use the correct rivet set, first one didn't set the rivets very well 2. Use masking tape to protect the primed surfaces 3. Be wary of where your back riveting plate is sitting beneath the surface. Scratches can be cleaned up with a little bit of primer in a cup using a q-tip