I am currently rust bluing a barrel with LMF solution. Just about to card after the 3rd cycle. It does work well, and I am satisfied with the results, but it does take patience. The last rifle I did, I was trying for a grey patina, and only did 2 cycles. But this one I am getting close to black, so I have a few more cycles to go.
I card with denim, rubbing quite vigorously once out of the water and cooled. I also use just plain tap water out of my water heater which is boiling, but I do not recommend this, since your water may vary significantly in mineral content. I use a PVC pipe with one end capped, and just pour the water over the barrel. I know this sounds more like scalding, but I leave the barrel in the water about 10 minutes, and get a nice black crud that has to be carded off. I don't have a picture at the moment, but will be posting pics of the completed rifle in a week or so, and you will be able to see my results then.
Matt