By hand with a circular motion creates no problems, and does a fine job on my stuff. Use plenty of water (one squeeze from saturated for me) and rinse them frequently and you'll be fine. I do most of my sanding with abralon by hand and I get very consistent results from ball to ball and on the same ball with touch ups on a weekly basis.
"...do or do not, there is no try."
St. Louis USBC Sport Bowling
Best Bowling Pro Shops Inc., St. Louis MO.