frockc,
The lower the grit the sooner the ball will want to roll up, having polish over top of the sanded area just allows the ball to get down the lane more. If you feel you need an earlier read on the lane, but also need strong backend sanding and then polishing is not a bad way to go.
Often the smooth finish that ball companies put on bowling balls is not enough for a bowler. So the bowler will sand the ball down and put a slight polish on it in order to get the ball to read the lane sooner, but the polish also allows the ball to be stronger at the breakpoint.
Goodluck
-EX-
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C-G Pro Shop (owner/operator)
Youngstown Ohio
Track Intl.-Amateur/Pro Shop Staff