I've never sanded mine, but urethane is notoriously HARD. I have sanded several and they usually need every level of intermediate grit to get the end point you want, a good deal of abrasive and pressure. Suggestion: be careful of the spinner motor; I know you're experienced. Be careful anyway, as we all are tempted to put a lot of pressure on these balls.
Oh, and the finer sanding of urethane produce much less grab than resins do. They go longer and hook a lot less when you do that. Again, you probably know, so I just reminding you. Unless you already have it at 2000 grit, I wouldn't go all the way to 4000 grit. Stock is 1500 grit, I think. Why not try 2000 grit + polish first?
Love to hear what you do and the results!
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"None are so blind as those who will not see."