In the following video I show how  make a custom dirt map in MARI and how to use 3D studio max with vray.
1.Create UV geometry to be exported in MARI
2.Esportare as OBJ
3.Import in MARI
4.Create channel spread with 2 levels
- Base level - white color
- Upper level - BLACK color brush
4.Create the painting of dirt in areas designated by you
Note: You can create custom brushes in Photoshop and import them as
5.A the end finished the painting export channel mode "Flatten"
6.The your map and 'ready to be imported in max
PS: I recommend you to use MARI to complex geometries
For most of the geometries' simple enough to use photoshop as in the next video.
Spero vi sia utile.