Countersunk the fuel tank attach points on one spar. Instead of Van's suggestion of a sawed off #8 screw as a countersink depth gauge, I made a dimple gauge from the same thickness aluminum as the tanks (0.038) after calipering them. Worked well. Pic 3 shows all the shavings. To avoid chatter, I drilled at a fairly low rate. I also tried 'bursting' the drill, which worked OK.
