Anyone else see this? It happens when I use either acetone or isopropyl alcohol. With either chemical, I dip a toothbrush in it and rub the PCB until the brown flux disappears. I then rinse with the board water and blow it off with compressed air.
I always get a white residue left over. Could it be from the water I'm using? Perhaps the bristles of the toothbrush (they look fine to me)? I'll go buy some distilled water to check but I'd be interested in hearing your opinions in the meantime.