Utilize Flux and alcohol. Clean with alcohol utilizing a wipe-clean-wipe method. After soldering with Flux, clean off residual Flux with alcohol. Leaving Flux can promote corrosion. My phone keeps autocorrecting Flux to capitalize it, and I don't care enough to fix it. 🤷♂️
Edit: Do not use clinical isopropyl alcohol solutions (~70% whatever). Those solutions need some water to work as a disinfectant. You want as little water as possible (>99% solution).
Believe it or not, isopropyl alcohol leaves residue behind. If you've ever wiped a shiny metal surface with it, you might see it looks a bit duller. Wiping after using alcohol is attempting to remove the residue to give the cleanest bonding surface possible.
I couldn't remember what the standard industrial solution was. All I could remember is that it's impossible to have 100% because water is introduced to the solution the moment it's exposed to air.
It's not impossible, just difficult. Depending on how water free you actually need the result do be, you might be able to get away with just a dehumidifier. If that is not good enough, you can put your entire purification process in a box and flood it with an inrert gas like argon or nitrogen. Storing this is not that difficult, but you need to be sure you are in a moisture free environment whenever you open the container.
Another interesting difficulty is you cannot direct distil it to beyond around 90%, because at that concetration water and isopropyl has the same boiling point. So, you need to mix in another chemical like benzene to distill out the water, then you can distill out the addidive as a second step.