Can anyone tell me what this effect is called and how it's created? It's in the primer as I sanded a bit to see. It's a 1961 Westfalia steel caravan which needs a complete respray and welding and felling in places so I need to get it all back flat then recreate the mottled effect before top coat. Closest I can think of is stone chip like under wheel arches but it's not quite the same.Thanks