I mean as far as stopping rust, you have to cut it out. Otherwise, no edge primers/rust converters/rust neutralizers will stop it... it will always come back.
But if you are crafty and daring if you cut out the rust and use duraglass and fibreglass mat/mesh you could remake the part.
It may be more work than its worth.
Bondo holds water, that's why it is considered a body filler, to smooth out contours, rather than fixing rusty holes.