Anyone here tried this stuff? http://youroil.net/rustshield_wax_based_.html

I'd like to use this in the doors,rockers and all other hollow cavities.

I've used it on a project. It's basically the same as the cosmoline you hear so much about. I don't have any long term experience, but I like the idea of it. I say go for it.

My 75 was treated with something simular, I presume from the dealer.
Dirt adhears to it, but it seemed to work well as a rust protection product.

It's a corrosion preventer, not treatment. So you've got to resolve any corrosion issues before you coat it, rust never sleeps!