I did hundreds of rust blue jobs over the years,
The process is a simple one as described above.
Less than perfectly clean surfaces, tank, application equip, hangers, carding supplys, ect will spoil a job every time.
Water that has minerals in it will leave a spot on the surface where the last drop evaporates as you pull it from the tank,,a small area of dried mineral deposit to spoil the finish.
Old oil and grease dislodged from in between the ribs of SxS bbls,,from screw threads, corners and crevises or parts will contaminate the water immediately and cling to the metal surface as you pull them from it. Then you card the part and smeart hat contamination all over it and your carding wheel (if you are using one) and pass it along to the next cycle.
Clean the tank before you use it,,clean the parts so there's no contamination on them. Pour out a small amt of your rusting soln into a shallow container to work from and NEVER pour any of that back into the main bottle. If you do,,that main bottle will be contaminated sure as hell..
I change the water in the tank after every 3 rusting/boiling cycles. It'll get dirty/brown looking and any contamination will be cleaned out at the same time.
I collect and use A/C condensate for my water. I try to collect around 60 to 70 gal durring the summer for use during the rest of the year. Some say the condensate running off the copper coils in the A/C leaches copper into the waterand spoils the bluing, but I've never had any problems in 40 yrs of this.
Some have municiple water that works fine,,it'll tell you right off. The red rust will either change to black and easily card off or it won't.
Rain water can work as can melted snow, both depend on .location and contamination in the air and collection methods.
As far as rusting methods,,hot humid summertime is nice,,but we don't always have that. The best and easiest rusting cabinet I've found is the bathroom. Hang the parts from the shower rod,,turn the shower on hot for a couple minutes with the tub stoppered to collect the water and close the door. Steam up the room. Turn off the water and leave, close the door behind you.
Rusting time 1 to 2 hrs depending on the steel type and the soln you are using. Check on the parts after 1/2hr and if needed turn the hot water on again the steam things up again and leave.
You might want to let the others in the house know whats going on first.
I don't plug bbls either when rusting or when boiling.
I do coat the bores w/shellac for damascus finishing as they have to go thru an etching dip as part of the process.
320 grit polish is about right for most jobs,,don't let the things rust too long. No need for a heavy red rust to form to get a blueing coat.
In fact I check them by feeling the part very lightly with my hands, I slide my fingers down the part and if I can feel the roughness of the rust coating even though I can't see any substantial rusting,,I pull them down and boil them. It avoids the heavy matted finish and you still get the layer of color.
That's part of the trick if there is any to the high gloss rust blue seen on the English guns. That and lower humidity and weaker rusting solns.
Anyway,,card completely, let the carding wheel do the work and don't push down too hard. If you use steel wool to card,,clean it of all the oil that placed in it for rust prevention before use. It's tough to remove it even w/ acetone. Some use stainless steel=wool. I've always just used carding brushes(hand) or a carding wheel.
If you do encounter a streak or blemish in the blue don't panic,,take a piece of clean scotchbrite and polish that area down till the blemish is gone and the area polish lines match the surrounding ones.
You will have taken most if not all of the blue off of that blemished or streaked area in the process,,but go ahead and put another coating of soln on the entire part.
Don't try and put extra on the area you just thinned down to bare steel,,,just a plain even coating over everything.
There is still rust blue in the pores of the steel in that thinned out area and this one new coat will bring the blemished repaired area back to if not full matching color then very close to it. A second full coat may be needed to fully match it up and then you can continue or quit right there.
I use Laurel Mtn Forge for slow rust bluing. It does have a tendency to after rust. So soak the parts in a weak alkali soln after wards to kill the weak acidic ph soln that it is (ferric chloride). Even then it can be still aggressive.
The best after rust preventative I've found for LMF is one coating of express rust blue right over the top of the last coating of the LMF. I use Mark Lee's Express Blue. For some reason absolutely no after rust.
Hope this helps.