Hi Charlie: In my experience, you need to remove all rust before priming. If you don't, the remaining rust, however small, will break the surface on the new paint, allowing moisture to get under the new paint, create new spreading rust and then you will get to scrape and repaint.
Sand blasing is an effective method. But better yet is to remove the railings, take them to a metal coatings plant, where they can dip them into a solution that removes all old paint and rust, then quickly dip them into a primer, let them dry and give back to you ready to paint. It isn't cheap, but over the long run I think its worth it, as you will have to repaint far less often.