Replacing old soldered joints. Two plumbers gave me two different answers...

[Sokolq55]

When “melting” an existing copper solder joint, how important is it to clean every single last bit of the remaining solder out before soldering that joint again?

One fella tells me as long as the new joints fit together, no need to completely clean out the solder. Another tells me I need to get 100% of the solder cleaned down to brand new copper. Maybe both are right.

 

[Breplum]

I am speaking of belief, a bit of experience:
Without getting back to bare copper, you would be dealing with two different solder alloys trying to bond, which is not conducive to a bond.
I have seen bad joints result if not all the solder is cleaned to bare copper.
We only use Viega ProPress now to mate up with old de-soldered joints, so never have to worry.

 

[Sponsor]

 

[Reach4]

Not speaking from experience: if you wipe the old melted solder, and it coats the inside of the fitting evenly, I don't see a need to go to bare copper. If I thought going to bare copper was important, I would tend to use a new fitting.

If the wiped surface has voids in the solder coverage, I would at least brighten up the voids to bare copper, and brighten up the areas of the copper that is tinned with solder too.

 

[Terry]

I have often soldered a pipe that has been used before but not the fittings. I sand off the pipe to make it smooth, flux the fitting and the pipe and make the joint.
Even if the old one was 50/50 from the 70's, I've not had a problem with my lead free.

 

[Fitter30]

Clean the joint to shiny solder, flux and haven't had any problems.