Looking at this picture is the bit of black in the ground mask, corrosion underneath the green mastic. Do I need to sand this entire area?

The area that already has mastic loss is from the glue or insulator material the factory used underneath the battery holder.

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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?

Thanks for the highlight!

Didn't notice any burn...the discoloration around the holes is the alkaline damage...I haven't treated it yet with Zep. The backside was fine, so I desoldered most components from the back.

I have a fiber pen for the small stuff, but for an area that large, what grit paper do you recommend?

Good idea...I did start with some 150 grit...slow going but it came off....I'll use the dremel to make it more square and presentable.

This board already had some previous repairs and lifted/damaged traces, so I figured it's a good board to practice on. Plus nearly everyone is out of replacement DE MPU's. I have a spare RD for the game until I get this one fixed.

Have some traces to repair with hookup wire. What's the best way to secure wire to board to relieve mechanical stress at solder point between wire and old trace? Hot glue or something different?

Thanks!