You can alter the surface during practice. That is perfectly fine.
Competition begins as soon as the first ball is thrown for score by any bowler on the league (not necessarily your team.) Once that first ball is thrown there is no altering of surface permitted.
From the USBC rulebook:
When may I alter the surface of my bowling ball?
You are permitted to sand, polish, or clean the surface of your bowling ball prior to the start of competition. However, once tournament or league play begins, altering the surface of a bowling ball is strictly prohibited. This includes balls that have not yet been introduced into play.
When does competition start?
Competition starts once any bowler throws a ball for score in a league or tournament.
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