This is a short step by step for painting on an explosive blast mark. It’s based on a mortar shell impact on concrete. It could also represent acid spit/splash corrosion with some green or yellow mottled into the scarring. My base color is unbleached titanium, which looks like it falls somewhere between screaming skull and ushabti bone in GW paint. The brown is burnt umber, it looks like it could be between rhinox hide and mournfang brown.
Here is the visual reference I’m using:
1. Body of scarring laid out in 1:1 base color and brown. I try to keep it irregular.
2.pitting added around main scar in same color. Most of the pitting is pointing in direction of main scar.
3. Mixed black and glaze medium (using Golden brand satin glazing liquid) 1:1 and traced top edges of scarring. Also added some to top half of main scar in from edges. Pitting has a black dot or line on top edge. A few more pits added with black+medium.
4. White line added to bottom edge of scarring and pitting.