I got an old AM-FM pocket transistor radio which looked good and clean
till I opened the battery compartment. Very corroded carbon zinc
batteries were in it. After removing them, I cleaned off as much of the
corrosion as possible by scraping with a plastic stick, and scrubbing
with q-tips and rubbing alcohol. That got rid of most of it, and I was
surprised to find the battery clips are not badly damaged, but I had to
use a fingernail file (sandpaper strip) on the ends of the springs.

Better yet, the radio works perfectly.

But there is still a little of that battery corrosion still in there. In
all the years I've worked on electronics, I have never found a perfect
way to clean up leaked batteries. Is there some sort of spray or a
chemical that will dissolve or deactivate that crap?

Of course it has to be safe for the circuit board and components too. I
use the 91% isopropyl alcohol, so it evaporates quickly and leaves
little water residue behind. (Then leave it dry well before use).