Rust Reformer 10.25-Oz Spray
Rust-Oleum Rust Reformer Stops Rust turning it into a sandable, paintable surface. ...
At the same time, it provides a great primer coat.
This is very simple to use--just spray on and you have a paintable black surface instead of rust (though of course you don't have to paint anything).
It's a very easy way to stop rust from spreading.