When I have all the geometry modelled and mapped I bake the Mental Ray ambient oclussion map, I usually end up using that map as a dirt map, this map is very very important because I use it as a base to fake the GI, reflection masks, specularity masks and transparence masks, (when it comes to cristals and windows). The mentioned "baking" can be done with the "render to texture" in Max7, which is finally compatible with Mental Ray
Ok! lets think that we dont want or we dont know how to bake the ambient oclussion map (maybe you have not Max7) what we can do is use it on the diffuse level slot of a ONB shader (making the roughness 0)
We can combine the layers in Photoshop, underneath the color, then the baked AO, in "multiply" mode and in top, in "screen" mode the capture of the UVmapping which we can obtain thanks to the free plugin Texporter
The next task is to include the textures that I have choosen from the colection of 3dTotal, in the mentioned layer of color making them overlay with UVmap capture from Texporter, normally I also use photographics maps in grayscale which you can find them in TotalTextures Volume 5th, include this layers on a top layer so you multiply them. Naturally you have to make a lot of touch ups with the Photoshop brushes by hand, so you can paint non mathematic dirt, cracks, and something like that in the building, I also highly recomend a graphic tablet, I use a Wacom Intuous 2 A5
It is very tough to make the textures overlay perfectly with the model if you dont have good continuity on the UVs, what I mean by this is that if the uvs have not seams you could always use Bodypaint so you can paint directly into your model or use the clone stamp as well. I usually bring the model to bodypaint to make the final touches adding dirt in those places where the math calculation of AO does not happen, for example, the oxide rake that leaves the rain under the iron elements
To light the scene I decided to use a target spot light very far away from the models with area shadows on with a yellowish-orangish color, I activate the ambient lighting on Vray with a little blue color, I also activated the GI with some predefined values (I think the preset "High") on the irradiance map.
As always, I modeled the shadow casters with the help of the "light" viewport (Shift+$) so the shadows are projected exactly in the desired place
Finally I start the render at a 3000 px resolution and adjusted little details in Photoshop, like levels and a couple of brush strokes in the sky as a "final touch" as always I conditionated all the work as a result more approximate to an illustration where the focus is the beauty of the form and color instead of an accurate representation of the building.
All the textures of this image were taken from the huge colection of total textures of 3dtotal, the color maps, the bricks, wood and concrete as well as the dirt maps. I hope that this has been usefull, to make old buildings is really amusing, enjoy it