What process are you using?
I follow Brownell's instructions on acid fume blue. The process utilizes two acids placed in shallow dishes in an enclosure. (I think nitric and hydrochloric, but check) Start with 5-6 drops each, adjust only if necessary to get a reaction on the metal with out too heavy fumes which can remove blue down to white metal--so more is not better. Fumes are heavy and will not speedily leave the container, but keep the lid on. My enclosure is an unused blueing tank lined with polyethylene sheeting with a flat plywood top also sheathed in poly. Wood soaks up an inordinate amount of the fumes if exposed. Poly soaks up a little but then no more. Some addition of acid may be required to replace that which is soaked up in the saturation phase. I support the barrels, etc. on the razor thin surface of the caps of spray paint cans, turned upside down, keeping the barrels suspended off the bottom. Seems to leave no mark. Barrels must be degreased completely, then boiled with a little TSP, boiled at least once in clear water to clean everything THOROUGHLY. (this is after any other metal prep, of course) I use thick cotton lined heat proof rubber gloves to handle anything from then on out, and keep them clean, especially of any source of oil. Stainless steel blueing tank is dedicated to boiling and it has Brownells hangers to support barrels off the bottom. ONLY USE DISTILLED WATER, though clean rain water may be OK, especially for initial boilings. Plug the ends of the barrels without fail while they are in the fume box. You will find a black fur on the barrels, which will come off as powder after boiling, removed by 4X steel wool which has been degreased with acetone or similar clean evaporating lacquer thinner type. Remove plugs when boiling. The bore will be exposed to boiling water but will evaporate immediately due to the heat. Ignore it until the whole process is finished. Plug, fume, boil, card off residue about 6 times or so, until the blue is deep. It will also be tough!! You will notice less and less reaction to the fumes as every pore of the metal is filling progressively with black oxide. If you cannot get much more reaction from the fumes, what makes you think raindrops will be able to rust it?
The finish will be matte, more than polished bright, but it will really last. Polish up the bores, then oil and you are done. BTW do not touch anything used in the boiling phases with anything oily, or you will have to degrease or replace said rubber gloves, etc. when it's time for the next job. This is a no bare handed touch, degreased process start to finish else you will have to start over from scratch if you put oily finger prints on it before it is completed. Put everything up clean and then oil when in your opinion the whole job is dark enough and done.
Otherwise, rust blue is simple, ancient and pretty. And an absolute must on ANY soft soldered double barrels! (hot blue will eat the soft solder)
