I made a sample coupon of a dimpled #6 screw and used that to countersink the wing access plate holes in the wing spar. There are only 12 of these holes so it was much quicker than the fuel tank countersinking. After countersinking, I used a Touch-n-Prep pen to apply alodine to exposed surfaces.

Then I un-clecoed the rear spar and doubler fork/plates and finished the dimpling in the flanges that I was unable to previously reach. That was followed by some hole and edge deburring, then surface scuffing with a Scotch-Brite pad, then cleaning and priming.

What's next? I think riveting the newly primed rear spar to the ribs.