Mk III actions are 1st generation attempts at MIM. I have heard them described as thin steel over soft iron. They aren't comparable to any action with machined parts. They shatter when they break. No gunsmith who knows Colts will touch the lockwork on a Mk III with a stone, or any other tool.
And again, there is nothing "coarse" about the surface of the lockwork of the Trooper. It's very slick. At least mine is.