Confused. If you used electrical (cored) solder, why would you add flux paste?

The fluxes used in electrical solder are traditionally resin-based and the best solvents to remove them are alcohols, as Martin suggests. In the good old days that would be trike (Genclene etc) or IPA nowadays. Nowadays the production fluxes are water soluble and increasingly "no clean" fluxes are used. They leave a small residue that is almost inert although if you insist on trying to clean them off with solvents, they will come away and contaminate the board.

The whole idea of fluxes is to remove any surface oxidation, which tends to require an acidic flux. The MSDS for this Fry's flux suggests it comprises "nonylphenoxydiglycol" as a surfactant (wetting agent) and hydrochloric acid to do the work. It's horrible stuff to get in your cuts and strips chrome plating. Best stick with the cored solder if you can get away with it for small stuff and keep Fry's for plumbing?