You want to stop rust, use POR-15. Nothing will get through this stuff. Be sure to use disposable bruches because they will be shot after using this stuff. I once tried to clean up a braush with paint thinner, no luck. I even left the brush sit in gasoline for over a week, and it still was hard as a rock. Short of cutting out the rust, POR-15 is your best bet.

__________________
1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member