#2. Re: How to un-solder copper joints. heat the joint up till you see the solder is liquid then pull and twist the pipe from the fitting. When you re-install a new piece of pipe heat the fitting to re-soften the solder, flux pipe to be inserted and push it back in as you heat the fitting and re-apply a bead of solder.e: How to un-solder copper joints. You are going to have better luck with propper equipment and material. For under 1-1/2. I use a turbo-torch stk-9 with mapp gas.