I buy it in quart bottles. I like it. IF you live in a low humidity area the slow rust process requires a sweat box or other humidity enhancement. That can be tricky because it is easy to get condensation and ruin the job. So, I use BC Plumb brown. IF you degrease properly, don't over polish, and don't slop it on all runny, it works very well.
Like any other finishing process you have to do it right. First use good ventilation, a big fan moving air to the outside in needed. BC contains nitric acid and nitric acid mists are bad for your lungs. OK, on barrels I plug the muzzle and attach an handle to the breech plug. I sand blast the barrel lightly. I degrease with lacquer thinner then alcohol. I use one propane torch in each hand to heat. I also use an infrared thermometer to check the progress. 250-350 F is a good place to start. I apply the solution with cotton cloth working from a plastic bowl. Use a wood stock to hold the cloth, like a big Q-tip. Difficult places get applied with scotchbrite and a little scrubbing. It takes several passes to get it dark and even. Weather permitting, I place the barrel in a humid place in the garden under some grape vines to rust further. I card with degreased steel wool, and if it looks good, I oil. There is about three after rust cardings after oiling, so don't put it together right away.
I heat and dip small parts.
Just an aside, DO NOT make any DIY quick blues that contain mercuric chloride. They work great but, MC is a highly toxic and cumulative poison. It will cause serious long term health problems.