renewing surface of pen

Are you asking about a fountain pen or a pig pen?

An alternative to keeping the natural color might be instead to powder coat it.

Pen or pin? Is this something like a fancy ball point pen with a metal body, or a metal quill, or what?

If you're just trying to refinish a metal-bodied writing implement, then I suggest that you sand it down again until you like it (long strokes down the length of the pen should give a nice visual effect), wipe it down with mild solvent, like 99% isopropyl alcohol (which you should be able to get at the drug store in most English-speaking countries), then paint it with clear lacquer ("Deft", in the US, is a good rattle-can lacquer, you can get it at Home Depot). The lacquer will keep the metal from corroding or getting dirty.

Be careful that it's just not some cheap metal-plated plastic thing -- if it is, then chances are good that it won't dissolve in the alcohol, but will be attacked by the solvents in the lacquer.

Heck of a question you've got there! One needs to know more than 'metal' to start; 'natural metal surface' presumably means metal without any patina? Most folk find some kind of surface finish appealing, and the easiest finishes to maintain are those that occur naturally with age (and can be created with various treatments).

Paint, polish, bluing, and purple hue on shakudo are very different, and any of those could be your best answer.