Rust in body seams is a bitch..if u have a drill buy a small wire brush attachment and clean out as much of the brown as you, take it back to metal if you have to..
Then get a product called rust converter and soak it,let it dry. Soak it again making sure it gets right down in the seams..its pretty toxic stuff so watch where it leaks..
Let it dry for a good week or so..wipe it clean then spray over it with fishoil, let dry for another week or so then paint..any paint will do really..killrust is just a brand Name, it doesn't actually killrust..
All this stuff is available at supercheap/bunnings etc..
But most importantly before you do any of the above
MAKE SURE YOU HAVE FIXED THE LEAK!!!!
Water usually comes in around tailights and windscreens, sometimes rust holes fan form under hoot seals and leak there too..