I was thinking POR-15 and bought a tiny starter kit. Not cheap for what you get. Summit catalog has a bunch of rust converters and sealers but looks like for different applications.

I saw a thread where GYC sends their shells out to be dipped, after seeing rust in every seam I get that now. Blasting a shell will miss rust in seams and inside channels.

In my teens I did mechanical work at a '40s Cadillac business, and I have a '39 LaSalle that has nowhere near the rust of these things.