Why would any one want to tin the outsides of the barrels ? Apart from the practical points that the blue/black/brown would not take over tin or that unless the ribs were brazed then surely dipping them in a vat of molten tin would loosen the ribs .Only possible reason that I could see for doing this would be to tin the barrels before the ribs were laid. Even then the practicality of doing so is beyond me as the cleaning off of the unwanted tin would outstrip the time saved in this method of tinning over the more traditional ways .