The easiest, best rust effect I’ve ever used, Modelmates Rust Effect, is now available again as Dirty Down Rust through Oakbound Studio.

Applying the rust is easy, simply shake the bottle, apply with a brush and watch the rust appear as the solvent evaporates off. Some great guidance for its use can be found here: https://www.youtube.com/watch?v=WcHufzVR5Dk