Here's a simple shader setup to add dirt or grime to your clean models.
In the .zip file is a folder containing a clip file with the shaders set up on the model, plus a stripped down building model and the necessary textures and dirt mask.
To use, unzip the file, open a new TG project and save it into the project folder. Under the File menu, select Insert Clip File, position your camera so you can see the building (it is located at 0,0,0), and render.
I stripped out my building shaders but left the Surface Layer nodes for them so you can easily add your own.
