Originally Posted by lime
.... The manual only mentions the cleaning and seasoning of the surface.
I would do exactly as the manual says. As Goodweed noted about cast iron, the seasoning process will fill in the grinding marks and you will have a perfectly smooth surface. And, since your manual instructs you on seasoning it, I would assume some minor "imperfections" in the surface are probably normal.
Unless you've got the proper equipment and experience to polish out any grinding marks you could wind up with an uneven surface (little dips and waves) that might be mirror smooth but would cause you more problems than you started with. And, I am sure, would void your warranty.