Well, it's kind of difficult to say how to polish this piece. The surface looks pitted to me, but that may just be on the photo. If it's actually pitted, there isn't much to polish. I would probably sandblast the piece in that case.

Otherwise, a magnetic tumbler with needles would do the job. These are terribly expensive, or could be easily built at home: http://www.youtube.com/watch?hl=en&v=3_FpiTzVYP8