I use to fine-tune my thumb hole (normally, I take urethane slugs or try the natural hole) with a half-round key file, because I need to smooth off the area where the thumb/palm interection is located. A smooth bevel also works best for me, because I have long and slender fingers.
Sanding the thumb hole with a grey or white 3M pad (800-1000 grit) gives me the finish that works best for my grip/skin/release.

I also experinmented with higher grit finishes, but they were too smooth and I had to tape the hole for secure grip (white tape) in the downswing and to prevent a too tight grip just to hold onto the ball.

You should experiment and find out what works best. Personally, I'd say that the thumb should rather be a touch too big to ensure a safe and clean exit than too tight. If it feels loose (especially in wintertime), you can always put some tape inside.
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DizzyFugu --- Reporting from Germany
Team "X": http://homepage.mac.com/timlinked/
"All that we see or seem, is but a dream within a dream..." - Edgar Allen Poe


Edited on 10/13/2005 4:54 AM