Quoted from balzofsteel:
Multicore solder beats Kester all to heck.
You are correct, sir! I like the Hydro-X. The flux is water soluable but it also cleans easily with a Q-tip with a bit of alcohol.
You guys using the stuff from home improvement stores and big-box stores make me weep. The flux in that stuff contains pure, undiluted evil. The smell from the fumes is a little bit of the evil escaping every time you use it. When enough of it escapes, it becomes a monster hiding under a child's bed somewhere in the world.
"No clean" is not. Whatever you use, please clean the flux residue. Some of that stuff will etch the solder mask if not cleaned.
As many have mentioned, many of thees boards are old, delicate. Be careful what you use for repairs.