Refinished Rusty Railings Already Rusted!
Not an uncommon problem.  Most of those metal railings are made of wroght iron, which is a very soft steel.  When you sand them off and don't level the "pits", the rust will come back time after time.  It is absolutely necessary to clean out all the "pits" as much as possible, then treat the area with a rust converter before priming to paint.  Rust converters are available at all home stores and help change oxides to a harden non corrosive substance. Read the directions and apply accordingly.  Difficult problem, sorry, steel railings are problem children.