Naval Jelly is Phosphoric Acid in a gel. It all really depends on the metal. You may not be happy with the results. The best way to get rid of the rust entirely will be physical abrasion like sanding or sand-blasting. Some stores sell a rust "converter" which stabilizes the iron oxide to a paintable surface.

I've tried Naval Jelly on old Alfa Romeo rotor/dust covers and found that even after 10s of minutes it barely removed any rust from the part. Sanding was the only way to fix that. I believe the porosity of the metal is very important to the various acidic treatments.