I have a nice looking purple PCB from Laen's batch service, which I have assembled using rosin flux. I have cleaned the board as best I can with isopropyl alcohol, which gets rid of the tackiness of the rosin, and also improves the dielectric behavior. However, as you can see in the pictures, it leaves a white powdery flux residue which looks unprofessional.
I tried 1631-16S flux remover, as well as just blasting it with compressed air, but it doesn't seem to get rid of the residue. How do professionally assembled boards manage to look so clean?