Does anyone have the name of a liquid that bonds to rust and produces a hard black finish?
I found it by searching. It is more than a primer. It actually bonds to the rust. It somehow chemically...does something...I read about it years ago. 

A user must be extremely careful working with it. You don't want it on your skin. You don't want to drop a bit of rust into the jar. You don't want to double-dip a brush into the jar. The whole container could react and harden. You pour out or scoop out some into a separate container to use what you think you will need.

Oh...Duh...The name of the product...

It is called Permatex Rust Treatment. I used it years ago on my car. It worked well. You just have to follow the instructions and heed the warnings. I remember that it worked very well. It was much better than sanding, priming and painting. It reacts and somehow bonds to the rust itself.