Brass light fixture over bathroom sink is rusting. What to do?

http://homeguides.sfgate.com/can-use-remove-rust-brassplated-bathroom-light-fixtures-105921.html

I hope this works!

take down lightly sand spray paint desired new color

I assume you don't want to replace it. You could paint in with something like Rustoleum, which comes in many colors and gloss levels nowadays.

I would suggest replacing it.

Remove the fixture.Use a rust priming stain blocker, and paint the fixture

If you like it otherwise, why not clean it as well as you can and paint it using a sponge and whatever color paint you prefer. The dampness in your bathroom over many years has caused the rust spots and might be smoothed with a steel wool pad before painting. I'd take the lazy way of sponging it so as not to have to remove it which I think you'd want to do if you sprayed it. You could even buy metallic paint that looks like brass if you want that look.

There are brass cleaners you could try.

If you want to reuse it, clean off the rust with steel wool, or a metal brush, prime it with Rustoleum primer once clean and dry, and and then paint it any color you like (I recommend a Rustoleum spray paint). Be sure to protect any parts such as the socket with a plastic baggie and tape before painting.

P.S. I would also use a clear sealer spray paint to seal it after painting it, since you've already had a problem with rust. Thin layers of spray paint are best, so you don't get drips. Paint it outside or in an open garage.

Lots of great ideas. Thanks so much!