The Stovebolt Site Forums The Shop Area Paint and Body Shop Cowl disassembly?

I have some rust holes on the inner and outer cowls that needs fixin but am debating about how far to tear it down. Most of the inner and outer cowls arent bad but when I look down between them there is a hole in the bottom and some surface rust throughout (see pictures). My questions is should I just take care of the rust holes on the inner cowl and the bottom of the outer cowl or should I remove a large section of both so the interior can be sandblasted then install new inner and outer cowls? Thoughts?

Not too familiar with AD but I don't see any "interior" rust out. Try to stick to the rule of only replacing rust, not a whole panel.... except where it is necessary to cut a little more to attach a patch using a more practical/favorable method. I.E. land on a ledge/spot weld flange/access for welding/backer.

At least one of your pics shows a pillar/hinge pocket not a cowl.

Interior light or heavy surface rust can be: reach-in spot blasted, wire wheeled or just scraped. Vacuumed. Then apply Rustoleum Reformer spay-on or Ospho or any type converter or encapsulator you like. That's exactly what those products are for.
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I would just make a flat patch for that pillar using a left over piece from some other patch. I would use a thick used grinder wheel to round out that bad welded inside corner radius.
But most of this is from just seeing a few pics from a far away place. Your informed judgment is best. Many other members here with patch experience.

There are patch panels available for that inner cowl panel. You're on the right track there. Cut out the rust and weld in new metal. For the A pillar (below the hinge pocket) scrounge up a small piece of 16 gauge steel and cut a piece to match what you cut out to get to solid metal. Butt weld that in.

As far as the rugged weld between the A-Pillar and rocker panel, that looks pretty typical for the factory welds. I had my cab sandblasted, and there were still pockets of weld flux in those weld joints. Not quite birds**t, but close on the weld quality. If it's solid, a little filler will make it look pretty. Or like bartamos said, dress it up with a grinder.

We discuss the use of Ospho a lot. It goes back and forth, pro and con.
If there was ever a clear call for using Ospho, your hard to get to inner parts would fit that build. Get a generic spray bottle from Ace Hardware or where ever and spray the Ospho as far as you can reach from above and below. It is very wet and will end up running to where it need to be. No need to clean up or paint afterwards. It will do its job of converting the rust (iron oxide) to inert ferrous phosphate.