E.g., "C-Flux":
http://www.rectorseal.com/product-da...ux/dscflux.htm

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?

Thanks,
Bob