I haven't had a chance to try this stuff on a rib repair job yet, but Oatey No. 95 Lead Free Tinning Flux is pretty amazing stuff. I would probably want to try it on a set of junk barrels first. It does a great job of fluxing and tinning the joint which is so important for a good job. Although intended for lead free solders, it works fantastic with common 50-50, 60-40, etc lead-tin solders. They also sell it in small tins. I first used it when I plumbed my house, and it made soldering with the lead free stuff simple and flawless. I tried it in desperation on a soldering job a couple years ago when several other fluxes failed and most of my 50-50 was rolling off and splattering on the floor. This was on a job where getting the joint perfectly clean was nearly impossible. It worked perfectly. I think it may do just as well on a shotgun rib. But I would still try to clean the repair area as good as possible with thin abrasive paper and acetone before soldering. Rubbing a graphite pencil around the repair area prevents solder from sticking to areas where you don't want it.
