If the parts are made of aluminium then you can polish them with Autosol or a similar mildly abrasive metal polish. For such small parts use elbow grease and a cloth
The parts to be polished need to be bare aluminium though, so if yours have coatings/ plating on them then you'll need to remove this first. Wet and Dry paper (1000 grit upwards) with soapy water will work well.
Polishing is also a good way of building up arm muscles too