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I've got to reconfigure some old copper plumbing that was installed in the 70s, so almost certainly were assembled with leaded solder. It will be fairly easy to unsweat the old fittings, wipe the pipes and fittings that are staying in place, and sweat on the new stuff, but I'm wondering if today's unleaded solder will flow and mate with the old solder film left on the pipes. Planning on using Oatey #5 flux. It is almost impossible to cut the pipes back to clean copper.
I don't know if the old style solder would mix compatably with the new...
I probably would spend the time with a file and sand cloth to clean all the old solder off the old pipes and have a nice clean new surface to solder on with the 95/5...
No lead solder will mix fine.
I've pulled many fittings apart from the 70's and resoldered using "No-Lead".
My favorite solder is Bridgit
I agree with terry. I've pulled apart and re-soldered many old lines, I never even thought about the mixing lead/no lead thing until now.
Me agrees too!
The old lead will come off the pipe very easy with a little sanding cloth and a new fitting will insure that all the old lead is totally gone.