Get them as clean inside the ribs as possible before the bluing. Otherwise the crud will just appear during the process as shown in the pic and spoil everything.
A trip through the tank with any agitation possible thru what-ever weep-holes/drain holes you can get is best.
Even then as you pull the bbls from the water and in your first coating when you pull them from the water, note if anything including clean water seeps from the ribs anywhere.
At this point, all you want to do is avoid letting it do that. You can by tipping the barrels one way or the other to dump the water inside the ribs towards the end that they will drain out the easiest and quickest.
Trapped in the slow draining end, it builds up for an instant and then decides to leak out those pin hole spots in the rib solder job.
Those small holes are very common dispite the claims otherwise. In over 40yrs of doing this, I can't think of more than a few bbls that didn't have at least a tiny break in the solder joint somewhere. Try and work around them.
The large exit at the muzzle by way of the solder removed works great.
I learned that when I did mostly hot-rust bluing and you had to work fast.
I still dry them out with a propane torch. BBls stuck onto a wooden peg pointed muzzle upwards and slightly forward in a bench vise.
Gently heat the tubes and the breech especially till steam starts to emit from the lower weep hole.
Then you may get a few spurts of hot water, so a paper towel is at the ready to catch it and wipe it from the metal.
A rag with some clean hot water from the tank is usefull for wiping over those spots to clear any spotting from residue that may exist yet in the water from inside the ribs.
At this point it hopefully dribbles down the extractor stem hole and away but you have to watch it doesn't splatter and spot the flats or bbls.
When no more steam is showing,,you may still be getting some smoke. Give it the sniff test.
Don't over heat and ruin the solder joints. The water will evaporate at just over 200F. No real need to get above that very much.
If the tubes start to creak like a ship hull under stress,,back off. You're way over what's needed to dry them.
If you do find a spot in the blue during the bluing process such as caused by one of these leakers,,you can remove it with scotch-brite. I use a maroon colored pad,,sometimes gray to polish it out and blend into the rest of the area.
It will of course leave a steel grey area where the damaged blue came off. But the next coat will generally bring you right back even with the rest as the pores of the steel are allready filled.
No need to scrap the entire job and repolish generally. At least give it a try.
Throw that small square of scotch-brite in the trash as it has the contaminants on it that caused the problem in the first place. If you leave it lying around, chances are you'll grab it again in a moment of polishing need and ruin another job while trying to save it..
I've never used compressed air for anything in the shop,,just never had the hook up.
Some systems allow oil from the compressor into the air line, filters or not.
Something you very much can do without around rust bluing.
