Damaged Reverses of Slabbed Coins

The crux of the problem here is that there is a substance on the slab, and as of yet nobody knows what that substance is. All we know is that this substance is hard, and it is in a somewhat regular, and at the same time somewhat irregular pattern, in places.

The goal is to find something that will dissolve this substance so it can be removed, but not dissolve and or mar the plastic of the slab. The only solvent that I know of and that may, stress may, meet these limitations and is easy to come by and easy and safe to use is 91% alcohol recommended by Kentucky. There are of course various acids that may or may not work, but they cannot only be risky to the slab but risky to the user as well. So I wouldn't really recommend trying them.

The OP mentioned that this substance was capable of being scratched off, so that presents one other possible solution. Use a single edge razor blade, in a holder that is easy to grip, and see if the substance can be scraped off that way. If it can be scratched off, should be able to scrape it off. If it works, it may take several new razor blades to do this as they may become dull with use.

With care, it may be possible to do this without harming the slabs. Or at least not harm them so much that a polishing compound cannot be used to repair the slabs.

It could be that water alone may work as water dissolves many things, so that is worth a try as well. Or, perhaps water used in combination with a razor blade, or even alcohol used in combination with one.