Reply to "Prepping roof for rust prevention before headliner install."

Any of the rust removers or rust neutralizers will help (Loctite, Navy Jelly, or phosphoric acid aka concrete etch).  In this case it is difficult because it is a roof and the liquids tend to fall to the floor.  You can also sand it with coarse grit which might be easier.  If treated with a liquid it will still need sanding.  Once the rust is knocked off then it needs paint.  Spray is easy but brushed on POR works well as Jimmy suggested although it can leave brush marks.