The usual wet sanding with sandpaper of various grits to get rid of the zig-zags. Seat posts are generally fairly easy to polish. I don't go too crazy below where I know it will be inserted. Different people have their own progression, but I usually start around 400-600, depending on the depth of the scratches, and then progress toward 2000 grit. In this case, I might wrap the sandpaper around a straight edge of some kind and avoid going across the flutes to keep those edges as sharp as possible. Then finish with Mothers or some other polishing compound.
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