Take the Sacs and place them in a shallow bowl.
Pour enough vinegar into the bowl to wet the coins.
Sprinkle salt on them.
The chemical reaction will remove all the dark spots.
this is how my mom used to keep her copper bottom cook ware bright.
The outside clad on the sac dollar is a brass composite ...
This method should work as well as anything.
DON'T DO THIS WITH OLDER COINS
Cleaning using this method is only recommended for junk.