I used the search engine to find an answer to my question, but I didn't see one so hopefully this isn't redundant.

I'm trying to remove the coating off a piece of 1/4" plate steel (sorry, don't know if it's cold or hot rolled). It is a gray-ish/black-ish coating covering the whole piece (almost 4'x8'). There were some digitized numbers printed on the plate if that helps you understand what the coating might be.

I've been using a 7" grinder (5000 RPM) with a 24 grit pad but I'm not making much progress. Only where there was red rust from a wet tarp that had been placed on top did the coating come off and I can see the metal underneath.

Someone at the metal shop suggested acetone, but that didn't seem to do much either.

Any suggestions you can provide would be greatly appreciated. This steel is our hearth (in a modern remodel) and I want to polish it as much as possible before I put it in place.

Thanks.