Proper "hot bluing" requires a tank and chemicals/salts, some gunsmiths have this setup and it will result in the best, longest lasting bluing. Cold blueing can be done at home and there are various over the counter " cold bluing" formulas that will give varying amounts of success. Proper metal prep. is most important with any bluing as is following the instructions and not rushing the job. I've heard that Brownells 44/40 cold bluing works good, and is meant for more then just touchups.