Start with your basic brick/mortar undamaged texture.
Then get a grunge map, define it as a pattern, then pattern Stamp it around, painting on a new layer in overlay or some other blending mode. This will get some nice grungy stuff started.
Then add another layer, and just paint in cracks and more specific detail like bullet holes, gouges, etc. Sometimes doing a little inner bevel/emboss effect on this layer for holes and cracks can improve the appearance of depth for those elements. Just a little bit though, or it starts looking really cheesy.
Add other layers for smoke and burn marks, and whatever else you need.
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Then get a grunge map, define it as a pattern, then pattern Stamp it around, painting on a new layer in overlay or some other blending mode. This will get some nice grungy stuff started.
Then add another layer, and just paint in cracks and more specific detail like bullet holes, gouges, etc. Sometimes doing a little inner bevel/emboss effect on this layer for holes and cracks can improve the appearance of depth for those elements. Just a little bit though, or it starts looking really cheesy.
Add other layers for smoke and burn marks, and whatever else you need.
2. import into photoshop
3. use various overlays of dirt from digital photos