Do you actually need separate flux when hand soldering?
I usually find that the rosin core leaded solder does just fine as it is. Some extra flux can be handy on heavily corroded parts, but for fresh from the box components and such it makes little sense.
Perhaps you need it when you're working with lead free solder, but there really is no reason to do that for hand soldering. Just re-tin stuff with ordinary tin-lead rosin core solder before use if required.
One issue with pre-tinned leadfree stuff is that it mixes with the normal solder adding extra tin so the mixture is no longer eutectic. With trough hole components like laser diodes this usually is no problem, but it can get a bit shitty with smaller smd packages.
word of advice - try not to inhale excessively during the soldering phase. It may make you unnecessary giddy [jk - it's not good fo ryou]. A breathing pattern in which you exhale when you solder is generally a good long term health-related practice.