Any sort of acid, such as phosphoric or even vinegar, will take off the rust.

It seems best to stop the reaction as soon as the rust is gone, or the base metal can start to be attacked by the acid.

I prefer to always remove all the loose or easily removeable iron oxide before the pickeling with acid. This can be done with tooth brush, dish soap, and hot water, and sometimes with a little hand wire brushing. Doing this will extend the life of your acid - no need to deplete your acid on loose pieces of rust that are ready to fall off anyway.

Bare pieces like this that stay inside the car can simply be waxed after derusting with good result. Bare pieces on the outside of the car can be painted with 'stainless' or 'detail gray' paint followed with a second coat of low gloss (NOT high gloss) clear.