Depends what you are soldering i use Carrs Green Label for soldering the steel droppers on my DG couplers,other than that i don't bother i just use 188 flux cored solder and make sure the brass or nickel is clean,this where the fibreglass brushes come into there own.Does it work well 50 odd locos built that way the oldest of which is over 30 years old and still running,i guess so Andy.