I've used Encapsulator, weld through primer, epoxy primer, what ever fit the situation. On the Burb fender well, I reached up between the inner and outer panel with Encapsulator and sprayed as much as I could. With the A project, everything comes apart. So a lot of Epoxy, then clean where you need to weld. Hidden/folded stuff got weld through.
To what Keven said, look inside your A pillar when you can. Or even far up under the dash. No protection and 70 years later, a layer of surface rust with no rot through.
