It’s the same as applying flux separately, the flux is displaced by the metal. If you’re applying fluxed solder directly there’s not much point applying separa

It’s the same as applying flux separately, the flux is displaced by the metal. If you’re applying fluxed solder directly there’s not much point applying separate flux to clean metal. Of course the flux will boil off pretty quickly so you need to use extra if you’re doing something where the solder wire doesn’t directly touch the components, or if the metal is old or previously soldered. The only advantage to non-fluxed solder for most hobbyists is being able to choose your flux, but for most of us rosin is fine and fairly non-toxic.