Actually, the GP2's finish, according to Track's website, is 1200 grit. I'd use either 1200 grit sandpaper, a white nylon pad with a lot of pressure, or Ebonite's Matte finish liquid, which is intended to finish a ball to 1200 grit. The closest Track's liquids come is the 1500 grit finish provided by Clean and SMooth.
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Bowling: Wish I could help me as much as I seem to help others ...
Life ... is what you make of it. No, really!