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If you have to use sandpaper, I'd use 2000-3000 grit, and VERY sparingly. If the contacts are plated with precious metal, you can bet that that coating is a few microns thick. The corrosion is more likely ome kind of spillage or contamination - even electrical grease that has attracted dirt and dust over time.
I crack up when I read that people think that they can make a fortune melting down old CPU chips like pentiums for gold. I read that National Semiconductor, a multi-billion dollar company, uses less than a pound of gold in a year in chipmaking. That's how fine the wires are and how thin the deposited gold is. Now, in pre-1975 computers, maybe another story.
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