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

I'm just in the process of replacing my upper cowl, not from rust damage but from dents mainly in the vents plus I wanted to paint in behind there and make sure everything is sealed

Like said above, this needs to be repaired from the topside to be done correctly. IMO I'd remove the upper and lower from yours and remove the doner one from your parts car by drilling the spot welds out from the bottom (not drilling thru the upper portion of the cowl) so you can separate the top from the bottom and only have plug weld holes in the bottom.

Then I'd weld the lower cowl to your car all along the A pillar and firewall seam, like factory. For the upper I'd glue the top and bottom seam with fuser structural glue and weld the sides/lead seam. Doing it this way will ensure you'll have no water leaks. The factory used a foam sealer and spot welded thru it (there is no such sealer I know of that you can weld thru) if you weld the upper cowl on you'll have leaking problems at the dash mount area which will leak inside the car and have problems with leaks in the engine compartment.

I'm not a huge fan of glue, I'd rather weld things but glue defiantly has its places and this is one area I recommend it. Fusor is very pricey and requires a special gun, but it's the best IMO

Here are some pics of my progress so far..













Hope this helps