There is a little corrosion here and there, but not much. I saw your mention of a dental pick in another thread, and so had already scraped at it with a pocket knife and then cleaned it up with alcohol, but you think that it might be worth going at it a little more thoroughly? I have seen similar things on a lot of old boards and this is the only one that doesn't work. But you know this stuff way better than me.
Is there is some chemical that would tackle corrosion (with a good wash afterward), or is a mechanical tool the only way to go?