For areas missing paint or where rust has gathered, sand the area good and smooth with a 300 – 400 grit sandpaper (start with a 150 grit and work up, if it is really bad).   Then spray the area with a rustoleum type spray paint in a can, you can purchase at any hardware store.  Be sure the paint is rated for outside metal surfaces like rustoleum is.  You should put on at least 3-4 coats, let dry between coats, (about 3-6 hours), and sand lightly with a 600 or 800 grit sandpaper in between the coats.   Do not sand the final coat.