The tinning process was done using an acid flux, which allowed the solder to adhere to the steel, on both barrels and the undersides of the rib. The acid flux was not something desirable if left in the void between the barrels however. Once "tinned" in that fashion, all of the acid flux was cleaned away, and rosin flux was then used to allow the two tinned surfaces to adhere. The rosin not quite suitable for adherence directly to steel, does work well for solder to solder application. This said, I doubt very seriously anyone ever completely coated barrels in actual tin. I believe that's just something that came about from a potential misunderstanding concerning the term. It's a term commonly used by sheet metal workers and gunsmith's alike to allude to the process described above, and other jobs where getting a good solder connection may be difficult. Cheap labor or no, I just can't see anyone subscribing to the amount of work required to remove all of the extra metal if they didn't have to. [Ask jerry what I went through relaying ribs on one for him. I've since figured out a few tricks to simplify the job, but removing unwanted solder was a complete and absolute PITA.] As the unwanted metal is filed off, both barrel steel and tin/solder acquire the same color and the two become almost impossible to differentiate for one thing. Also, it's easy enough to tin the complete inside portion of the barrels by applying acid only in that area, thus keeping any unwanted material to a minimum. I've seen the void on a few old SxS's, and most seem to have been done this way, as you can usually see a small spot or two that was missed. We're also talking about a time when labor was quite cheap, but materials were considered dear. Why use much more than required?
It's also become my belief, and that of others whose knowledge and background I respect, that not all "good" barrels are completely free of a pin hole or two in the rib solder from day one. After some years of checking ribs, I've found that some can have a pinhole or two, but still have perfectly sound rib attachment and no major rusting going on between the barrels. This seems to happen more often in older American made guns than in those of English/ Euro manufacture. Which is why I believe you see weep holes more often automatically,[it seems], included on many American made guns.
No intent on raising hackles, nor calling anyone or their friends/ relatives liars. I just think my background in the HVAC&R industry gives a little extra potential insight on the "tinning" aspect of this discussion. Feel free to have at me if you feel otherwise. I'm fairly thick skinned.[Others may say thick headed as well].

Last edited by JimfromTrafalgar; 05/25/14 10:33 AM.