The last parts in the Horizontal Stabilizer that needed to be machine-countersunk were the stringers. As before, I used my micro-stop drill tool to make these countersunk to allow the dimpled skin to later fit into these rivet holes. This countersinking had to be repeated for the two stringers.
Once that was done, I took the ribs and stringers and used a scotchbrite pad to scuff up the surfaces and then clean them with acetone. Then I sprayed a quick coat of self-etching primer and allowed these pieces to dry.
What's next? Time to start riveting together some of these internal parts.