Would something like this help you out?

Eastwood Internal Frame Coating Rust Prevention Aerosol

Destroys rust hiding inside the chassis, rocker panels and pillars so it won't rust through

www.eastwood.com

 

I used this on a few areas and was quite happy with the product, of course you can't get the Black spray right now. To clean up the nozzle if you want to reuse it I just sprayed brake cleaner down the part that attaches to the can and it cleans up nicely.

 

One doesn't. Its impossible to do so. That being said, very very few areas are inaccessible. Even the innside of the rockers can be reached using a wand that places like Eastwood sell

 

Make sure to clean the special cap immediately so you can use it on any paint can that has the same nozzle style.

 

I’ve thought about those. But, and maybe I’m over thinking this, how do I prep the metal properly? Or do I not need to with a product like that?

 

You can drill a small access hole in some places, spray what you want in it and then put a rubber plug on it (Ford used em all over the place) or plug weld It shut.

 

I just don’t do anything. If it’s light surface rust on a lot of these areas then it will be good for another 50 years if it’s already made it this long. And my cars never see water or salt.

 

Hole. 360" spray wand. Master Series Silver. Optional cavity wax. Rubber plug. Done.

 

Maybe this would help--?
3M™ Weld Thru II Coating
Part Number 05917

 

That's simply paint with a high volume of zinc solids.

 

You could use something like Master Series metal prep on the back side. While it won't last indefinitely, it would help with slowing down development of surface rust. The use of a good copper infused primer on the back would add protection too.
I was quite surprised when I cut out a section of my 70 fastback trunk lid. There was quite a bit of surface rust inside the structure of the deck lid. As mentioned, I used the Eastwood internal frame coating as there was some access and the hose provided gave me enough reach. Not a perfect solution but more than Ford did.

 

On my scout, which was already rusting on the factory floor! I drilled several 3/8 holes, and used fluid film and a wand in those cavities. Then I just used a 3/8 plastic plug to seal up the hole. It worked very well. The eastwood frame spray works too. I like the fluid film better.

The holes are in an inconspicuous area, and I'm the only one who knows they are there... I spray in there every few years, just for piece of mind. I'll use the same method on my mustang.