Seat posts and stems are ideal first projects because you can practice on the section that's hidden inside the bike.

I had that "what am I getting myself into?" feeling on my first project but that quickly faded. You'll doubt yourself, especially if you have to resort to more aggressive grits like 400. The finish goes a horrid dull gray and the part looks ruined. But with each finer grit the finish clearly gets better. The final polish with Mother's is magic.

On fluted seatposts, the indented parts were often not highly polished originally. They are a little bumpy. Try using just Mothers and no sandpaper. That may clean and shine the metal to your liking.