Drake Seta of the Battle Bunnies here. Thought I would share with you a technique I learnt this month from a friend to add a bit of detail to a flat plain armour plate.
First you need Martian Ironearth technical paint. Get the piece of armour you want to add the subtle rusting to (think logically about which areas to do this on, and once again less is more).
Paint it quite thickly on to area you want to show rust. Bare in mind that the thick layered part of the paint stroke will be the area that breaks up the most.
Airbrush / paint over.
Now a you need to do is add your favourite rust streaks and shades to make the rust stand out.
Look at car rust on doors etc for inspiration.