I've used a scrubbing brush (eg from floors) to clean a PCB that fell into a alligator tank, it was the temp probe to monitor things. It worked again afterwards, despite being tiny SMD.
If a toothbrush isn't cutting it, try one of those little brushes for scrubbing (under) nails. Much harder bristles, and easier to hold/exert pressure on to get out the junk right next to SMD resistors etc.