(I have read other "cleaning PCB" posts but I don't see an answer to this aspect of the issue)
I use a combination of solder paste and rosin-core solder (what can I say, I was given a bunch of it).
I clean the boards with 99% alcohol and it works well, except ...
After I have done several boards a sticky film is left behind which, I assume, is flux that was cleaned off but left in suspension.
I tried filtering the used alcohol but the filter material quickly clogged up and became useless.
Using several staged baths as someone suggested would help but is not a long-term solution if doing a lot of boards.
I can only think of two solutions:
1) Distill the used solution to remove the suspended flux (this assumes a lower boiling point for the alcohol) ... heat and alcohol - this sounds like a recipe for disaster ...
2) Use a centrifuge to remove the heavier (?) flux (isolate to the bottom of the container) ... complicated but doable