Today I finished removal of the damaged F-776 center bottom skin. Using the new skin as a drill guide worked great. You can tell from the first photo just how well the new skin centered the bit on the rivets. Using a #41 bit to drill them out left complete rings inside the holes. Most of the heads had to be snapped off.
The second photo exemplifies just how well the rivets form to the inside of the holes in dimpled sheet to really lock them together.