Scratched black metal

 

Prime black.

Start with Heavy Charcoal. Use a sponge on large surfaces, or a stipling drybrush on smaller ones.

Use a smaller sponge/smaller drybrush/lighter hand for the second layer, which should mostly be placed inside the previous patches. This is a darker grey, which I obtained by mixing 3:1 Heavy Grey with Heavy Charcoal.

Use an even smaller implement for the final stipling of Cold Grey. Keep it extremely light, and inside the larger patches.

Use a small brush to paint some Stonewall Grey scratches by hand. On larger surfaces, this will take the form of small lines placed in a cluster, or sometimes selective edge/corner highlighting.