I updrilled the elevator horn and elevator root ribs. It all looks pretty good. Since I will be using the AeroConversions spring trim system, I will not need a trim tab and the associated brackets, anchor nut, etc. This will give the airplane a cleaner look (and a hair less drag), and is simpler. I verified with Sonex that if not installing the trim tab, none of the flush rivets or other differences between the left and right root rib attachments are needed. That means I can just use regular CCP-42 rivets on both sides in all 17 holes per side identically.
I am ready to rivet, but the plans call for touching up the epoxy primer on the steel horn before riveting; I have asked online and one answer for the correct primer to use is NAPA 7220; I'll pick some up tomorrow (since today is Sunday and NAPA is closed), and spray, then rivet the following evening to give the primer adequate time to dry/cure.
