This is how I clean mine now, take a look at the YouTube video and try it out on yours.
https://youtu.be/yZe0ZQavIzU
I like the results but your coins are coming from much better soil conditions to begin with. I've recently found eleven Indians at a church under a massive tree with roots everywhere. I'm guessing the chemicals in the tree roots sped up the corrosion. At other sites I have found crusty indians very near to nice ones. Who knows?