This tutorial is aimed to 3dsmax users but can be achieved with any 3D package. First I will show some details of the modeling and after I will explain my texturing process.
Part 1 - Modelling
There's nothing really hard to model in this scene. I add some details on the objects when they're mapped to match the object maps. That make easier the modeling and mapping process. 3Ds Max is nice for this type of work, you can modify the mesh with an UVW modifier at the top of the stack, and when you toggle show end result, you can model the details with the map as guide. I used renderable spilne to make the shapes on the windows.
Part 2- Texturing
General texturing tips
For most materiasl I used the Oren-Nayar-Blinn shader, because it has diffuse level and diffuse roughness channels that allow more control on the diffusion of the color, that's very helpfull to had dirtiness, hide the seems of the texture or darken some part of an object (Raytracing and multi-layer shaders have also diffuse level and roughness channel)
The uv channels are very important and I use them massively to manage my differents textures maps and masks. I use differents uv modifiers with diffrerents channels( 1, 2, 3...) to manage my maps, I rename each uv modifier with the name of the maps corresponding, example: UVW bricks, UVW dirt, UVW metal...
To easely tune maps, luminosity, contrast and color I use the Color Correct freeware plugin
I use often mix maps and blend materials with masks, it's a good way to get complex mapping with a few basic maps.
I will show the making of two materials, one complex and one simple, it represents the two kind of material I used in this scene. To achieve the differents effects of my maps I used a lot Photoshop to composite dirts maps.
The Steel door
The steel door is a composite of a base color map and some dirt and graffitis layer blended to add details. The dirt maps are blended in multiply blending mode and colored with the Color Balance image adjustment. To make the blend of the graffitis more realistic I erased the paint where the pieces of the door meets each others. To achieve this effect I used the color range selection, I selected the darker areas and I erase thecolor of the graffitis with this selection.
The Brick Wall
For the brick wall I started from a screen shot of the mesh in front view to easily place my details. For the diffuse and bump I simply used a clean brick texture. The details and dirt were add by the diffuse level and diffuse roughness channels of the Oren-Nayar-Blinn. I made it by compositing in Photoshop.