Rust is a chemical reaction, O2 trying to combine with FE (iron) to make iron oxide (rust) FeO2.
You must neutralize any rust because the O2 in the air will get to it and start it all over again.
Wire brush, clean with solvent, converter, prime and paint.
Take it from a salt water boater. I know corrosion.
2007 WK Limited Hemi QDII
1998 ZJ Laredo 4.0 Selectrac