[T77911S]

i would clean as much of it off with a wire wheel, then pour lots of OSPHO on it, i think advance calls it something else now, let it sit for a few days, clean it up again then put extend on it.
ospho is osphoric acid, great stuff. it eats the rust. extend does too, but not as good. the extend puts a better coating on top to prevent it from rusting again.

if there is something newer or better out now, someone will chime in.