Coating on fast hitch parts?

ShawnAgne

501 Club
About done with the project and cleaning up the fast hitch. What I'm noticing on all the pieces is under the paint there seems to be a hard brown layer on top of the cast iron. Wire wheeling doesn't remove it, but sand blasting will knock it loose a little. Any idea what it is? Wouldn't think the previous owner would have done this. It's not anywhere else on the tractor.
 
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