The buttons get sticky with age. What you're feeling there is a combination of oil, dirt, and dead skin from people's fingers. In many cases you can restore them by simply cleaning them with alcohol. You can also coat them with Plasti-dip and that makes them look new too. I used the Plast-dip and I'm happy with the result.
That didn't exactly answer your question but I hope it helps.
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'99 black 986
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