Inaccessible rust areas - how to protect ?

[Trevor]

I'm getting ready to blast and paint all my chassis parts and I'm wondering how exactly to clean and then protect the inaccessible places, permanently.

A few example areas; between the frame and all it's reinforcing layers in/around the top of the rear coils, inside the cavity (boxed area) at the bottom of the radiator core support, gap between the trailing arm halves etc. etc.

The last thing I want to see is rust dribbling down from these spots everytime I give the truck a good wash. I have a feeling most people don't bother but, I'm OCD about things like this so ...

So how and what can I coat these places with ? I have some ideas but ...