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

You could try wire brushing the older wire to expose some bare copper then flux it. I've never had a problem solder older wire to newer wire........I soldered tons of wires. I also try to heat up the older wire first to help "suck" the solder in that direction. If one wire has heavier strands it does make it more difficult though but if you put the heat on the heavier wire it will still work out. treblig