Thanks for your suggestions. I have added scratch textures to my diffuse map in overlay mode. Do I do the same for my normal map?? @Jack: Must I convert my diffuse map into normal map after I add scratches? Because it is almost impossible for me to do the same two strokes with brushes.
Here is how my weapon looks like now. Just the same texture applied on both normal and diffuse map. Now how do I increase the depth of the same scratches?
Usually, some good brushes that add grime and scratches into your diffuse (and accentuated in your normals) combined with specs to really bring them out will be adequate to make your weapon look like it's really seen some service.