When changing the coverstock how long should I sand each side either going to a lower grit or higher grit when using a ball spinner ? Is it possible to "over sand" a ball?

Just make sure you do the same amount on each side to keep it even.

Spinner is best, and make sure to sand the ball from 4 or 6 sides, and each side for the same amount of time, same pressure, same revs, with plenty of water. This makes sure the ball stays round!
Start with a rough grit and work gradually "up" to the final fine grit.

You can hardly "over sand" a ball, but remember that you actually take away material with each sanding step, and that the way you sand and the grit steps have effect on what you get for the ball's reaction.
At one point, the ball can become too small(!) and illegal, and on the thin-shelled modern offerings I think that frequent and thoruough sanding could have negative effect on the overall ball integrity, promoting cracking through stress.
And use, if possible, new or lightly used abrasives (like Abralon pads or sanding paper) for good results - and lots of water to avoid dust and clotted resin in/under the sanding material.
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DizzyFugu - Reporting from Germany

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Say you start out with 4 sides on a spinner with 500, if you wanted the final surface at 4000, would you use a that on a spinner or sand lightly by hand?
And, how long would you hold the 4000 grit on there, because you don't want to cut to much into the under surface, just round off the tops of the ridges.
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titletowncards