They can be polished, but it's a long laborius job. As the propeller is marine-grade stainless steel 316, it needs a relatively hard compound to remove the surface rust, before working towards a mirror finish. Under no circumstances use steel wool, metal files, etc. All you will do is transfer ferrous material onto the surface of the prop and increase corrosion. Cleaning stainless steel encourages a fine film of chromium oxide to form on the surface when exposed to air effectively sealing the surface.
For best results you'll need a bench polishing mop, but a hand-drill and mop can also be used.
To start, wash down the prop with warm water and a 3M Scotchbrite pad to remove any surface corrosion. Rinse and dry. Don't use mild detergents (have salt, etc). Use fine-grade abrasive papers if there is still surface corrosion. This is a useful site for polishing compounds. I've used Menzerna products before - and can recommend. http://thepolishingshop.co.uk/acatal...Round_Bar.html