When soldering pots, the main thing is to be sure the pot is clean, no grease, oil, even oil form your fingers can mess us the adhesion. I use a 100 watt iron and standard electronic instrument solder, the high quality stuff. Can't recall the specs. When heating up the spot I'm working on I keep the iron on the wire and apply the solder from the side. Once I get a good bead built up, I turn the iron off and keep it on until I can see that the solder is cool enough it won't let go when I remove the iron. Not necessarily the text book method, but it works for me. I don't use flux on pots.