I managed to get most of it off the first few figures I decided to start on. However, the Witch-King had a stubborn chunk that I couldn't shift. I think I managed to get it think enough that I can disguise it with paint.
The Cave Troll was a little more complex. The hammer came loose when I was scrubbing it and removing the many, many layers of paint showed that it had been put together in a fairly slapdash fashion leaving lots of gaps: