Originally Posted by supcom

Pick a drill bit about the same size or larger than the back side of the rivet (The side that is mushroomed out. Drill through the front side of the rivet and the rivit will fall apart. The front side is the large flat part with a small hole in the center.

You can use any ordinary electric hand drill. Although it would be easiest to remove the fenders from the bike, you can probably get by by removing the wheel and using a piece of wood behind the fender to stabilize it. Watch your fingers and guard against the bit slipping.