For carding I use a roll of new canvas tent material about four inches in length and two inches in diameter. I can really rub hard without doing any damage. This yields a fine grained rust with no scale.
I use a damp box ala Bivens, applying every 2 1/2 hours carding between applications. If I start at 8 am with a polished and degreased barrel, I am finished before I go to bed, usually. If it isn't perfect, I remove it from the box and leave it on the bench 'til the next morning. It is really dry here, so the damp box works well, though without it one week will produce the same result, applying only once a day.