After doing some research, I determined how to dimple the end rib holes (machine countersink). As such, I countersunk the end rib holes as well as dimpled the skins. Although the countersinking of .032 aluminum is less than aesthetically pleasing, it is workable for 6 rivet holes. Also, I completed the pop-rivet holes for the rudder (rib ends and rudder brace). After completing the pop-rievting, I noticed that the lower skin/horn attachment was not clean as well as the rudder horn pop-rivets. I did my best to re-work this area (sanding/met massage). That being said, it does not look great, but , as the issue is 100% aesthetic, I chose to leave the area as-is.
