Years ago I purchased a thick substance/liquid in a small white plastic jar from Sears. It was a thick off-white liquid that one would paint onto a rusty metal surface. The liquid would change to a hard black finish that was quite strong. I cannot find it now. Does anyone know what I am searching for? It is not Rust-O-Leum. It is not a paint. It actually bonded to the rust.
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.
Por-15 does this too.
Do not get it on your skin: it will not come off easily.