What comes to mind when polishing out machine marks is, the diameter of your seat post will be reduced around .006" or more, that may pose a problem.
None the less this is how I would do it. Start with 150 grt emory cloth with a slight twisting motion and reversing the directon of sanding that will leave a cross hatch patern" this will help maintain roundness" work your way to 300grt emory paper. Next, use wet/ dry 600 grt with a penetrating oil [WD 40] Finally use Mothers metal polish on a flannel cloth. It would take me about an hour doing it by hand.
Here is an example of metal polishing I do on a more difficult surface, a motorcycle heads combustion chamber. Using the cross sanding method, no bumps or lumps.
http://s170.photobucket.com/albums/u...eadflow003.jpg