Hello AnyNameWillDo,
I use Acetone (Nail Polish Remover) to get off heavy flux using an old toothbrush.
It cleans it quite fast, and I've never had the solder-mask or print on a PCB that
got smeared or leeched off.
I then give it a clean with Isopropyl alcohol to get rid of any residue.
Using alcohol alone on heavy fluxed areas will take you ages to remove.
You will find that the toothbrushes or any other brush will eventually gum up, but
soaking them in acetone overnight cleans them reasonably.
Furthering Ian.M's suggestion, you can purchase a dispenser like the one shown below:
Dispenser Even if you knock it over, the only thing you'll spill is what's already in the upper bowl.
They are excellent for that reason. The liquid will also keep, provided you keep the lid
closed, it will never evaporate away.
Regards,
Relayer