Even though you/we refresh balls on a regular schedule, still lane wear does change the surface, especially on very fine surfaces. Wether or not your surface is a 500/4000 or a 500/1000/2000/4000 sequence, you should do a resurface from scratch every 30 - 50 games depending on the wear.
Since you do regular oil extraction, it might be a good idea to do the total resurface at the same time.
Several months ago, someone from Rotot-Grip came here and said their newest covers should be refreshed every 7 - 10 games.
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