Oily rag

Here's a Sunday poser for you. How would you replicate the effect seen on this miniature traction engine, on a model ?

I'm talking about the oily sheen with dirt hanging on around the boiler bands an rivets. In real life this is simple - wipe the item over with an oily rag, buffing to a shine. It's both clean and dirty at the same time.

In theory the trick is to work on a gloss or shiny satin paint job. Then wash over with some black, presumably sating finish. Finish by wiping the black off with a cotton bud or similar to simulate the cleaning process.

Trouble is, this never works for me. And I've tried many times. What I end up with is streaky dirt and generally matt dirt at that. What I want is the effect of paint that is being seen through a translucent oily sheen. And an idiot-proof way of achieving this.