I think it is best (that is, the image will in the end seem more cohesive), if you work over the entire image on each step, rather than pick a single area, detail it to completion, then choose a second area, and so on. All of the separate areas will need to fit well with one another, since this is a single image of a single thing, so treating them as if they were independent might not be the best route. I prefer to block out my shadows and highlights (using maybe three colors total) first, and then continue to refine and add colors as I progress.

As for the finished product, I think you're over-dithering, especially on what should (I assume) be a smooth, metallic texture.