I would suggest something like:
I never use added flux for electronic soldering**. If you need flux then the parts are not clean enough (i.e. old parts that have been sitting around for many years). If the circuit board does not readily accept the solder you should clean it with a scotch brite pad (do not use steel wool).
BTW the reason you should use 63/37 instead of 60/40 is the 63/37 has little or no "slushy" phase. It goes from a liquid right to a solid. This makes it very good for parts that could move during the cooling phase of the solder.
** No clean solder has flux in the solder, it should be all you need.