My latest addition to my paint collection is a mould effect from Modelmates. It's not quite the same as the rust effect but it's still pretty cool and now that I've figured out how to use it right it looks great. I used it on Kranon (new name still a work in progress) and while I slightly messed up in one area I figured out how it works and there's another area I absolutely love which is on the inside.
It works best when I brush an entire area in one sweep instead of lots of small strokes. Small strokes looks patchy and unnatural but long strokes gives it the linear look of real mould and it's pretty cool.