Yeah, ask questions and they will be anwered excaustively
Several ways to add dirt. PF straight into color input of default shader gives UV based variance, taken through transform shader set to final/world, it will be world based. You can add a surface shader after the default as well, with children and mask (same priciple apllies here), but now the dirt/color is over the default shader so can cover all, unless coverage is decreased. Displacement can be added here too. Another way is to merge the default and surface shader into the parts shader, and control by world pf. Then they exclude eachother based on that mask. I just used that to make a ruin with partly plastered stone walls (stone texture default mixed with plaster default by very hard merge control, both with their own displacement/bump).
