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I've soldered new copper pipe but have never resoldered it.

A shiny band of silver solder remains on the end of the copper pipe after an elbow has been removed from it. Does that shiny band have to be sandpapered away, to get down to bare copper, before flux can be applied and a new elbow installed?

This is 1-1/2" copper drain pipe (DWV), if it makes any difference.

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The main problem, if any, assuming the solder band is complete and without flaws or gaps is if there's too much solder on the pipe and you cannot insert it into the fitting. The pipe is "tinned" or coated with solder, and there's nothing wrong with that.

The usual approach to making sure that the coating is thin enough is to wipe it down (while the solder is molten) with a rag - easiest if done when you heat it up to take it apart, but you can heat it up again and do that if the fitting won't seat properly as it is. That should leave it thin and nearly mirror-shiny, which is what you want. Caution is needed as you are dealing with hot, molten metal.

The basic secrets of soldering are that things should be physically clean (shiny metal) chemically clean (use appropriate flux) and not overheated (too much heat burns off the flux and solder won't wet the newly-oxidized surfaces.) The silvery solder is perfectly fine shiny metal. Dull gray solder needs to be cleaned up, but there's no need to go to copper.

  • Good to know. I've always assumed one must remove the old solder and "start over". So I sand... – DonBoitnott May 8 at 17:23
  • I know in the electronics world that contamination of lead free solder with small amounts of leaded solder can lead to significantly weakened joints. I don't know if this is a problem in the plumbing wolrd.. – Peter Green May 9 at 1:57
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No, as long as it's well-bonded and clean. Flux as normal.

Some will say you'll have a higher chance of success with clean copper, but it's the difference between 97% and 99%, especially since this is a non-pressurized joint.

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As the other answers for clean, but to re-do joints like that I use a brand of flux called "Power-flux" and it's more aggressive than ordinary flux, works a treat for those joints that don't "take" first time.

Do remember to clean it off well after.

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