Making something look grimey, or used-and-not-cared-for is a nice weathering effect on a prop.
For a basic grime effect I would start by pouring 1 ounce of silver paint into a cup, add two drops of black paint, mixing them together, then adding in some paint thinner to dilute the mixture (very watery.) Now just Wet-brush the solution all over your prop and let it dry (also known as adding a Color wash). Voila, instant grime!
You can use other colored paints to simulate other grime also. Diluted green paint gives a patina or Copper-tyle grime, possibly simulating mildew. A Yellow wash of color would make the prop look like it was from a stone or desert area. I would add a couple drops of silver paint to the mixture to create an "organic luster" to the grime.
Once dry, take some of the silver paint and use a DRY-BRUSH technique to simulate paint chipping on the edges of the prop, not much, just enough to look like it's been banged up a bit and the paint has chipped in some places. Use very "light and minimal" strokes to get the desired effect. Test your technique on paper before applying it to the actual prop too.
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