Try \"scenic rust\" :
a bit down the page here
It\'s real rust made from micro-ground iron and an accelerant to induce the rust over 24 hours.
We use it a lot and its excellent, you can vary the effect by controlling how you apply it and how much accelerant you add (or rust paste/powder).
Can be applied in tiny amount onto individual 25mm scale nuts, or over an entire tank/building:
bunker
It seems expensive but it lasts a LONG (long, long, long) time and, being real rust, can\'t really be beaten for effect
I believe there\'s an American version similar to it called \"Rustall\" although I have not used that one.