These cases, damaged by water, proved to have only about 0.001" penetration by the worst of the corrosion after cleaning in the ultrasonic. It takes serious corrosion to actually eat through, though I have seen it. Always was verde gris (the green corrosion) present when it happened, though.
Don't use a polish with any kind of ammonia in it. I recommend a little diatomaceous earth on a damp cloth. A little toothpaste will give you that. Wipe it off afterward. Just don't let the damp part get on the primer.