Having done dozens of these jobs, my favorite carding is with a wire wheel. Mine if from Brownell's. The wire size is 0.0025". 0.008" would be way, way too stiff. I 've also done dozens of jobs with degreased (it's important to degrease it) 0000 steel wool. Either will work fine and neither will scratch the surface of the work. If you polish to worn 400 grit and don't rust too aggressively you can get a beautiful, soft satin finish with either carding method. As in all rust bluing it's critical that the surface of the work be absolute clean so that the first coat goes on evenly. If you get major streaking on the first coat even many additional coats won't even it out. It's also important that you get no oil in the tank as this will coat the parts when you take them out so wipe the bore as clean as you can get it but don't degrease it.
My favorite, by far, rust bluing agent is Pilkington's American Rust Blue that Brownell's sells. It's not cheap but one bottle will do a couple dozen jobs and it doesn't have any mercuric chloride in it. Most of the browning solutions are too aggressive and if you aren't careful will pit the surface. But then that all depends on what you want, rust slowly for elegant finishes, rust aggressively if you like your guns to look like they were neglected and left out in the rain for a week or so every year.
Tom
