I've done bluing for several years and i quit it. it's not worth the effort. bluing is least forgiving towards process and and least durable of any finish.
This is hot bluing I'm talking about. cold bluing is even worse than that. honestly it shouldn't even be called bluing, since it has as much in common with hot bluing as does cola with beer.
but I'll share my former setup and absolute minimum you will need to get a good hot blue finish onto steel.
bluing tank made of steel as long as the longest gun you plan on bluing. half a gun at a time while will work but will leave a very noticeable evidence where two separate bluing sessions met. stainless is the best but non-stainless is also doable, just expect a lot of rust particulate on the finish and very laborious post bluing hand cleaning. drain is a must have for the bluing tank. bluing salts are very hydrophilic and will absorb moisture out of the air very quickly and will literally crawl their way out of the tank in the matter of couple of weeks. if you tend to not do bluing often, draining a tank and storing solution in a bucket with air tight lid is very much advised.
pre-heating tank. since you will be bluing in a relatively small capacity tank, pre-heating of the action is something you need to do. since dunking a cold gun into hot solution brings down the solution temperature rapidly and changes the color of the bluing from blacks and into brown and even plum shades.
degreasing tank. hot degreasing is better than acetone dunk it is especially necessary if you are bluing things like AK actions, where individual parts are joint by riveting. those areas tend not to get degreased with just acetone bath. heat is needed to squeeze out oils out of the captive areas like tiny space between receiver and trunnion and from under the rivet heads, etc. if you don't remove that, it will seep out in the hot bluing tank and ruin your bluing job, by preventing bluing from taking where oil was still present.
blasting setup (cabinet, big compressor and blasting media) - a way to remove old finish and/or apply uniform finish. this is typically done by glass bead blasting. glass beads leave nice sating finish that is pretty uniform and pleasing to the eye. it will not completely hide deep scarring or rust pitting or machine marks but it will make them less noticeable.
unlike Parkerizing (phosphating) bluing doesn't hide minor imperfections nor it adds layer of finish over crappy surface (like paint does). whatever crappy surface part has pre-bluing it'll be same crappy surface post-bluing just brown/black in color. therefore if you are looking to get mirror or black chrome like bluing you will need to use all/any required grinding and polishing methods/practices to create that perfectly ideal look prior to applying bluing to it. bluing is not a shortcut to that old school good looking finish it's merely one of the many steps getting there.
- surface prep - is a single most important step of the entire bluing job. if you screw this step up you will screw up ENTIRE job. proper degreasing and handling of item to be blued is what separates good result and time wasted.
this sort of experience is best grasped over time through trial and error.
