I thought it was time I did some prop work, so I decided, in light of the release of the Avengers movie coming out soon, that I would do Mjölnir, Thors Hammer. It also helps that I am a massive fan of Stargate and of Thor and the Asgard in particular
A combination of grunge brushes, and use of layers and blending. The key is to make the scratches and dirt look real by applying it where you would expect to go. So dirt, in all the crevices and recesses, and the scratches, on all the exposed edges. Then bring them out with contrast in the specular map, you can also add more emphasis on the scratches by duplicating the layer you make them on, and adding them over the normal map, this is also how the engraving is applied.
Hope that helps.