Basically, you need to polish it but that means different things depending on how bad the finish has gotten. If there is heavy oxidation, my first step would be to buff with a wire wheel or wire cup on a grinder. Continue with finer grades of wheels and eventually get to buffing with a felt or even finer cloth polishing wheel with polishing compound. Experiment in a small non conspicuous area first.
If the finish is not too bad, you can try wet sandpaper. The grit to start with will depend on how bad the finish is but needs to progress up to a pretty fine grit like 1500 or even 2000.
Don't skip too many grades of grit. If you start with 400, go through 600, 800,1000 1200 or 1500 and maybe 2000. Otherwise you will have scratch marks. All that can be done without power tools but it will be work
