I use pencil and Tippex as a barrier to prevent Silver Solder going from its intended location. I hope you understand, its hard to put words to the problem. Would Black Graphite stove polish be as good or better ? Many years ago old fashioned plumbers would use "Plumbers Black" to prevent lead based solder going to where it wasn't wanted, the wiped joint looked very tidy, under ground cable jointers did the same. I have been trying to locate "Plumbers Black" for quite some time without success. I notice how neat some of the professional made boilers look when compared to home made ones, not to say that both work equally as well.Alot of expensive Silver Solder is wasted, what do the professionals boiler makers use ? John