Hi,J. I'm sure that somebody will give you their favorite home made recipe for copper and brass polish. For something like this, a buffer with jewelers rouge is about as fast to polish the copper as anything but try their recipes if you don't have access to a buffer. But don't forget that you can set one up on an electric drill easily also. When you use jewelers rouge, wash it off with mineral spirits to remove the black gunk and leave the rag outside for a couple of days.
I believe that I would use a spray clear lacquer on the vane before you installed it. Apply a couple of light coats. It dries fast so you can do a couple of coats in a day and there is no need to sand between them with lacquer. No clear finish will last forever in this application so you have to plan on redoing this every 5 years or so. Copper just loves oxygen and when they combine, you get the brown layer that eventually turns green. Poly or spar varnish will work but they have to be removed with a stripper. With lacquer, you just have to wash it off with lacquer thinner, polish, and spray it again with fresh lacquer.
I'm sure that you will get other suggestions and they will probably work too. But this is the way that I would do it.