I dont know what can say more about this technique. The principle is very simply. For imitation of concave volume of the scratches and dents need additional light and shadow, light strip on the bottom, and dark strip on the top. Think, main problem is the placing of the scratches. But think, the placing of it on the armour must be different, for example. legs will be damaged strongly than helmet, or outward part of the hip armour plate is scratched strongly then inner part. May be try to do scratches in more different size and forms. Think, edges of the armour plates will be very scratched. Oh, and one more, scratches in the shadow plases will be mush darker than on lightened places, such a helmet`s top. So, need several tones for drawing of the damages. Two colours, ligt and dark for all surface of the mini is not enough. Need at least two tones for lightning and two tones for darkening.