How would you fix this cowl rust?

Looks fine, the problem was a bunch of leaves on that side. When my grandma lived in Sunnyvale, CA (1980 to 2008) it spent all of it's time parked on the street under a tree. The crown in the road meant that whenever it rained the water drained out to the right, and the leaves eventually stacked up against the hat and rusted it out. The left side is perfect, and inside the cowl there is barely even surface rust. The outer side was dried by air coming in the drain I suppose. There are a couple of pinholes where the a-pillar structure is spot welded to the top of the cowl from salt air, but I'll fix those from the outside with the fender off.

Cool, all that matters is if your happy with it, I'd keep the cowl cleaned out and run a hose in there once in a while and it should keep the problems at bay. Maybe squirt some zero rust as deep as you can in the fender outlets towards engine after you clean and dry it. This is a nice tight area that likes to rust too. Steel is ok with the brush of water, but as soon as it stagnates with leaves on top of it watch out!

Nice work, good luck!