Rust Reformer stops Rust Cold.

At the same time, it provides a great primer coat.

OVERVIEW: Rust-Oleum Stops Rust Rust Reformer instantly converts rust to a non-rusting surface. Apply to rusty metal only. Compatible with most solvent or oil-based paints.   PRODUCT FEATURES: Indoor/Outdoor Durable, corrosion-resistant

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.