Bluing, be it hot/caustic or slow rust is all about the prep. No matter what solution might be used, poor metal prep will show. All corners retain their original shape, no dish-out screw holes, no pits, not even the ones you'd need a magnifying glass to find, all lettering and numbers intact and not 'washed-out', all polishing marks running the same direction and no marks from the previous grit, no ripples or irregularities in the polishing, work like that is getting harder and harder to find. And it costs when you can find it. Slow rust bluing takes more time, because it's not just a matter of submerging the parts into the bluing solution. You need to keep in mind, hot/caustic bluing salts are shipped hazmat, and they are heavy. It's not unusual for a shop that blues to order a couple of hundred pounds of bluing salts at a time, or a 55gal. plastic drum. Bluing salts are regularly added, so you can use quit a bit. It takes 7-10# of salts per gallon of water for the initial start-up, and they don't last forever. Sooner or later the 'old' solution will have to be discarded and new mixed. Which brings up another point, the 'old' solution must be neutralized or disposed of properly as it is hazardous material. Labor for prep, hazmat shipping and disposal, it all adds up.