Quoted from ForceFlow:
Yes, you'll need to sand the affected areas to get rid of the alkaline spreading across the copper traces. Untreated, it will continue to spread and corrode. Looks like you got lucky and the damage was fairly limited so far.
The damage wasn't too bad...down the left side to RA9 for the solenoid driver. So most of the blanking circuit. Two IC's, the 555 timer, about 10 resistors, a resistor bank, and 6 or so caps. Was in an Operator's game that had been stored for who knows how long. A shame too, because it looks like they had shopped the game before storage.
But back to my question, so I understand you correctly, do you think that mottled blackness in the entire ground trace area is corrosion? So sand it down to the bare copper, the entire quadrant?