Cowl Rust! What is the best way to fix it?

It's gonna have to be repaired from both sides since it has rust on both sides. If the inner cowl has rust that bad, you can be assured the under side of the outer cowl does too. There's really only one way to get it apart as redness described. Cut the bad out, weld good in, reassemble. I tell you what. I would slobber so much Rustoleum on that damn thing before I put it back together that it would add 10 pounds to the car. lol