Rust converters convert iron oxide into ferric phosphate by chemically penetrating the rust. Paint does not adhere to phosphate coatings, and most rust converters are also painting primers. Secondly, rust removers are chemicals that will eat away at the rust when left to sit for a set amount of time. This method of rust removal takes longer and requires an extra primer to seal the metal after the treatment is complete. Without resealing, the chemical process that causes rust will continue.