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

I have done this before. Cut sections of the cowl off to get to bad spot, then I got a parts car & cut out the section of the bad vent hole. Weld it, grind it, then use duraglass ( its water proof ) putty is not. Then I sand it w/ 80 grit. I also put on a coat of seam sealer, then brush on some rust preventive paint. If you weld up & putty the top section of the cowl you will never know it was done. Good luck & let us know how you make out.