Does Your Hot Solder Drool and Cover Your Copper Wires Like Hot Butter?

You need to scrape the old wires clean "as clean as possible" and you need some ELECTRICAL compatible flux..........NOT acid, designed for plumbing, etc. The electronics industry has a number of liquid fluxes, and one I always used to use is called "Nocorrode" (spelling?) which is supposed to be safe for electrics

EDIT "Nokorode"

nokorode for electrics - Google Search