There is a reason why this isn't a good idea, yet I continue to see discussion of sandpaper as an 'viable' option to cleaning the contact edges of cartridges.

Simply put, sanding roughens up the surface and destroys the gold plating originally bonded to the metal traces. While aesthetically ugly, the real danger are gouges in the metal that are then succeptable to oxidization and rust. So while there may be a stubborn spot or already rust in that next cartridge you inspect, the proper way to tackle such problems is to take a fine steel/copper wire brush, preferably on a dremel, and tackle that in a precise manner. If you can get a smooth, velvet brushed texture finish then you're good to go.

You could even re-plate the surface in gold if you had the proper equipment, which is something I'll be looking into in the near future. The gold plating itself is invaluable in that it prevents corrosion and provides better, more even conductivity.

By no means should sanding or steel brushing be considered a first or even second resort, never underestimate the removing power of a white erasure.