I use WD40. Spray a small amount on a rag. Never had a problem with my NES after that.
I don't see how a high grit sandpaper would be any more harmful than the people using minor abrasives like Brasso.
Once you get up in those high grit counts, they're about as rough as a cat's tongue.
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Imagine if you damaged the contacts enough to make them jagged or rough. It could damage the contacts on the system too then. baaaaad idea.
DERP
When I'm cleaning my games I use a lighter and some brillo..........oh no wait, uh.... I take that back. Um yeah.....
In yo face!