How do I get rid of the rust on my wiper arms by sanding to repaint them to a gloss black? What grit sandpaper should I use? What kind of paint do I need to paint these a gloss black? How do I do it?

 

There are so many angles to the wiper arms that I would not consider using sand paper. I would either have them soda blasted by a paint shop, use EvapoRust or simply replace them with new ones.
The most practical advise: don't bother!