Rust between body layers

[Geiri]

So this is the a photo of my old 1976 Body.
I know the rust goes deep in there between the layers.

This seam will be sand blasted, but what can be done more?

 

[jeffncs]

In my opinion…. See what happens after the media blasting. You may find that seam “disappears” with the rust which will force your decision to do the quarter and rear corner.

 

[Bajabrewer]

Rust converter is what was recommended to me by a trusted painter/body man. lots of different brands just make sure to follow the instructions on top coating/painting.

Here's one brand
https://www.amazon.com/dp/B000K21KR0/?tag=classicbroncos-20

 

[kancox]

I use a very thin dremel wheel 2 or 3 inch diameter diamond coat on both sides of the blade if i remember right i got it at menards and its for cutting ceramic tile. It cleans out the old seam sealer and rust very quick. Put wheel in as far as you can then when hit a spot weld pull out far enough to go around it and back in to the next one. Go"s very fast. Then rust converter and see what you got

 

[Broncobowsher]

Judging from the depth of the pitting next to that seam, that cancer runs deep.
What does it look like on the back side? That's the bed corner, with the tail light bucket? You should be able to peak inside and see what the back looks like.