E.g., "C-Flux":What's the advantage? You still have to apply solder to the joint. What's the difference if you plain-flux & solder, or C-flux & solder?
The linked web page says "Flows smoothly into even the smallest cavity". Is that the advantage - that solder flows into the joint _better_? I just had a plumbing job with about 60 joints. 2 leaked. Would the use of C-flux been better?