For rusted bolts/nuts, nothing I've tried works better than Kroil.
Phosphric acid is not a rust remover, it is a rust converter - used to get the microscopic rust in pits after visible rust has been sanded/ground away.
Several products on the market contain phosphoric acid - OSPHO can be found at NAPA stores and paint stores, PickleX (formerly Rust Mort) goes a step further than OSPHO does - if the car is in a shop, you sand/get rid of all surface rust and treat the surface, it can be left alone until time to spray. With OSPHO, you basically want to treat it, let it dry, then prime.
Navel Jelly is a common rust REMOVER, but you must make sure that you rinse thoroughly or you have problems painting over it.
Even if using a rust remover, use a phosphoric acid treatment on the metal before priming.