My wife recently brought home some stuff from one of her scrap-booking classes that mimics the effects of rust. It comes in three colors in the package, dark rust, red rust and gold rust. It has some grit in it to mimic the bubbling you frequently see on rusted metal. The stuff is called Rust Effect Paste by Finnabear.
I've only seen it on a couple of projects she's done, and on the plastic fork shown below. It does show promise, though, and I plan to try it next time I need to simulate a heavy rust buildup. Anyway, here's the terrible cell phone picture:
The glossy shine in places is because it wasn't quite dry yet. It dries flat, and it's acrylic-based.
Gene Beaird,
Pearland, Texas