How to remove the rust from the metal? ...
Answers: Martin,
If this a mechanical item or parts that entail the rust removed thoroughly before one refinished or painted, a muriatic acid dip will do a super duty. Muriatic can be bought inexpensively by the gallon at many lumber/hardware & pool supply stores, used surrounded by large subdivision in treating swimming pool hose.
Smaller parts can be immersed in a dip, using a spray bottle or brush to apply it on larger items. You will see a foaming/bubbling management occur as the muriatic cleanses the metal. Depending on the severity of rust, dipped items will be arranged to remove and rinse in as little as 5-10 minutes.
Be sure to read adjectives warnings and directions on the container- use merely out of doors, wear gloves etc. as this is a chemical that you need to use cagily.
What is it on? There are products like L.S.R. that should work on small job, and bigger jobs will probably require sand and refinishing.