How do i add grunge or dirt to a BSP with a material on it?

I am attempting to make a village that resembles a middle eastern/african white clay houses so far i made the basic shape with BSPs and added the sand stone material to them now i want to add a bit more detail by aging them but cant seem to figure it out. I am using 4.10.4 right now until i add new ram and go up to the 4.15.

dont use BSP for modelling, it just wont work properly.
Use meshes, this needs to be done in blender, 3dsmax, maya, whatever.