It sounds like a lot of barrels have at least one or two small voids in the rib solder joints that could allow moisture and contaminants to become trapped inside. I've noticed small streams of air bubbles on occassion too, but I've never done a rib re-lay job to actually see what's in there. It would seem to me that a few pinholes could be just as bad as a much larger void when it comes to having rust issues. But I've always been under the impression that getting the barrels hot enough to drive off the moisture as steam would leave the interior dry, and that the area between the ribs would typically have been completely tinned before soldering. Wouldn't this tinning offer almost as much corrosion resistance as if that area was galvanized?
I also would like to ask if anyone has any good recipes for homebrew rusting solutions that do not contain mercuric chloride. The last homebrew I used had too much bite. I suppose I could have diluted it more with distilled H2O, but it took about a dozen turns to get good color at the recommended dilution.
