With chrome, it shouldn't need polishing, just cleaning.
For other metals, wet sanding followed by buffing is the usual method.
Depending on how scratched the surfaces are depends on which grit to use when starting off. If you're unsure, start with 400 grit. If the scratches are taking too long to remove (more than 15 minutes of sanding by hand, drop down to 220 or even 120).
Sand in one direction until the metal is shiny and all the scratches (called "lines") are from the paper and all in the same direction.
Jump up to 600 grit and sand in the opposite direction until the only lines visible are from the 600 paper.
Go to 800 and switch directions again.
Go to 1000 and switch.
Go to 1200 and switch.
Wash and clean with soap and water.
You're now ready for buffing. Use a sisal or spiral wheel on a 6" buffing wheel with brown tripoli.
Wipe down with denatured alcohol and rebuff with jeweler's rouge and a flannel wheel.
Here's my stator cover after 120 grit, 1200 grit, and then buffing with the tripoli. I did all the steps I listed above, but I didn't take phots at each stage.
Total time on this piece was about three hours, if I remember correctly. More if you're working with steel instead of aluminum.