If you use a motorized wheel to card, typically it would be at slow speed with very fine wires. Too fast and/or too coarse will remove too much of the oxide that you worked so hard to achieve. Better to stick with fine steel wool in the beginning, and carding under slowly running water is easier and lets you see any spots you may have missed more easily. Use gloves or you will need to degrease again.
Very smart of you to learn and practice on an old junk set of barrels. You can practice on the same set as many times as it takes to get the hang of it. You might also want to try a couple other old junk single shot barrels before moving on to the big time, and then you will learn that sometimes you need to adjust your techniques or solutions or rusting times for different steels. Too many guys start out with grandpa's Parker or L. C. Smith and ruin a nice old gun with real potential. Sometimes you may be better off leaving remnants of deeper pitting rather than trying to get it all and removing lettering, engraving, or excessively thinning the barrel and leaving dished out areas. When you learn to do a good job, you will understand why the pros charge so much to rust blue or brown a set of barrels. The materials don't cost a lot, but it is time consuming and labor intensive. Good luck.
