I agree with everything Kutter said. A set of 28" barrels will have at least 116" of solder joints along both barrels for the top and bottom ribs. I learned long ago that it is common to have one or more areas that either were not perfectly soldered during manufacture, or became separated at some point. So the problem of contaminants getting into the voids between the ribs and barrels may have been going on for decades. It is possible or even likely that gun oils, barrel cleaning solvents, grime, and rain has gotten in there over time. Getting any of that garbage sufficiently cleaned out and neutralized before starting the rusting cycles is just part of the job. And despite your best efforts, it is possible that something will leak or creep out after one of the boiling cycles. So after you have done the best you can, making sure that hidden void is totally dry is important before doing the next application of rusting solution. Kutter uses a propane torch. I use the same burner that is used for my boiling tank. I pass them back and forth over a low burner flame to heat them as evenly as possible (to avoid differential heating and expansion). Just getting them hot enough that they are uncomfortable to hold is hot enough to vaporize and dry out any moisture in there. There is no need to get them anywhere near hot enough to worry about melting the rib solder joints. Some instructions advise drilling weep holes where none are present to facilitate this, but I haven't needed to do so. I have to wonder if Kutter is shaking his head over the Professor's question of whether a heat gun could be used instead of a propane torch!!!But both will contribute to Global Warming, so that might keep some folks awake at night.
It is important to look for any areas that may have leaks during the cleaning process done after old blue removal and prep. There may even be existing weep holes, and there will be a hole present if the front bead is removed. My first cleanings are done with Dawn Dish soap, and then I immerse the barrels in a hot lye solution to remove any traces of oil or grease. Then they are thoroughly rinsed and dried. While in the hot water, I look carefully for any small stream of air bubbles that would indicate leaks that may cause problems later. In the event that anything does creep out during the rusting cycles, I can usually control it by spot cleaning the area with degreased 0000 steel wool and denatured alcohol. This prevents spotting, streaking, or uneven areas.
When all rusting, boiling, carding, and/or etching cycles are finished. Everything needs to be cleaned, neutralized, and rinsed again, which means more moisture in this void has to be dried out. After the blue is well cured, it won't hurt to get some thin oil, Ballistol, or corrosion inhibitor in there to prevent future rusting. An existing weep hole makes this easy to do with a syringe. Of course, all excess oil must be drained out so it doesn't leak later and oil the stock wood. I've also read that some barrel joiners tinned the entire tube assembly before soldering, but it's hard to believe anyone in their right mind would do it more than once. It would just waste tin and flux, and create a lot of extra work removing the excess tin before bluing. The area between the barrels may have been sufficiently tinned to prevent corrosion, but it isn't possible to know for sure. Attempting to repair a small area of lifted or imperfect rib solder joint can be problematic. Soldering is very easy when you have a perfectly cleaned and tinned joint, the correct solder, temperature, and flux. The only rib repair I have done was fairly easy because it was loose at the muzzles, and I was able to get some very thin abrasive paper in there to clean the joint without removing the original tinning. I then flushed it with electrical contact cleaner to remove any contaminants. Trying to clean and repair a contaminated and imperfect joint can cause a frustrating mess of things, with much swearing and gobs of solder rolling off and onto the shop floor.
