The best solution is to have them powder coated.It's a little expensive but they will last ten times as long as a paint job.Look in the yellow pages for someone who can do the job.

On metal chairs use Black Star Rust Converter.
It will turn the rust back to metal. The surface will be a black primed surface. You can leave it black or paint over it.
The chairs will be rust proof for the life of the chairs. No More Rust!