I've used this for years with good results. Place a small amount in a generic household hand pump sprayer, i.e. cleaner bottle. Mist it on the rusted metal. Allow to dry 24 hours then coat with paint or other coating of your choice.
I believe its active ingredient is Phosphoric acid.
Phosphoric acid (also known as orthophosphoric acid or phosphoric(V) acid) is a mineral acid having the chemical formula H3PO4. Orthophosphoric acid refers to phosphoric acid, which is the IUPAC name for this compound.