Metal preparation once the initial #40 holes were drilled, was to, after removing the canopy, essentially enlarging the holes to a #6 diameter. That includes all the standard preparation, meaning , deburring , cleaning , dimpling when required. The only hole I decided to countersink rather than dimpling was the one in the middle of the aft structure attaching the roll bar. I also made a 1/2 hole in that 4" structure bar to give an access for the nut that will attach the screw.
The exception to through holes were in the roll bar where I drilled and tap all the holes to accept a #6 screw.
