getting rid of the rust

be warned that Ospho is a phosphoric acid product/ I use it and like it personally, , ( DuPont makes some similar products but are very expensive.) it does good job of neutralizing surface rust. BUT you can have a reaction to it when applying a paint or primer. test a spot first to see how it reacts. Be sure the Ospho is completely dried, needs to be on bare metal, NOT over any paint or primer.
I guess you are talking INTERIOR metal????? most bodymen ( I admit i am NOT one!),,,,do NOT like any type product where they paint. they sand down to bare metal, and use an epoxy type primer.