Hi Kevin ! Here is how you create a dirty map: 1- First get a white piece of paper and throw some black ink on it, yes, make a big mess! or get this white piece of paper put it against a rough wall and rub a black crayon on it. 2- Now scan this paper and you got a wonderful dirty map, like I did in the picture above. 3- When applying the texture on the color channel use the mixer option, source 1 will be the color of the dirty you want (dirty is not always black, it can be white if it's pigeon drops)on source two you use the beautiful texture map or color that you want on the surface, and on the blender channel you put the dirty map you just created. 4- Then you adjust the tile option to make the dirty proportional to the object you are applying it. 5- And to finished copy the dirty map to the bump channel, (Hold ctrl down and drag the map from blender to bump channel)this way the dirty looks like its caked on the object. 6- Congratulations! you got a disgusting dirty surface! I hope I could help you, if you have any questions, please fell free to contact me. Good Luck ! Marcelo