On May 27, 10:17*pm, Tim Wescott wrote:

It's "Top Line Quality Soldering Paste Flux". *Bought over thirty years
ago (I don't go through flux very fast). *It looks like bearing grease,
& it sizzles when the iron touches it. *Works great, but leaves a
residue

The paste flux used for plumbing is ZnCl2 in a vehicle of petroleum
jelly.
If that's what you have, any waterless hand cleaner will loosen it,
and you can flush with water. It's inconvenient to do this inside
a finished tank, so consider tinning the seam bits, cleaning the
flux off, then pressing together and heating without any extra flux.

Boiling water rinse is a good starting step. When most of the
petroleum jelly is dissolved, the ZnCl2 will dissolve in water.