Here I will show you how I create a painted metal surface worn by time with paint chipping off, revealing older paintwork underneath. I used a similar technique for the BUB model seen in the gallery.

Here's the model as a wireframe. 

After applying a uvw modifier and arranging in unwrap uvw I assign a grid texture to the objects. The grid quadrats will most probably be distorted so I manually adjust the tiling to get the right map aspect ratio and make the squares quadratic. Turning on "Show Map in Viewport" is quite helpful here. For this object I needed 1.3 tiles to 1 so the aspect ratio is 1.3 to 1. Now I take a snapshot of the unwrapped meshes with Texporter and save as a TIF file (TGA is fine too, just make sure you get the alpha channel). I wanted a map height of 800 pixels so the width would be 800x1.3=1040

It's not always neccesary to be this accurate but I do this to prevent getting maps distorted in one direction. Often you can get away by just estimating the texport aspect ratio as well. 

Here are the unwrapped meshes, ready to be painted on. Within Photoshop I start out by copying the alpha channel into a normal layer and set it to Multiply. This way the black wireframe will show but everything that is white will be transparent, allowing you to see what you're painting. This layer will be the topmost one and always locked to prevent accidental painting.

 I then put in a layer of yellow paint and one of green paint, using several layered photographic maps as a base and coloring them.

The result in Photoshop looks like the following picture.

Now with the wireframe picture as a guide I start painting in where paint is to be chipped off into a new layer using black and white, highly contrasted grunge photos with the clone stamp tool and some careful handpainting where it's needed.

I then copy this layer into the alpha channel. Using this as a selection I delete the chipped paint off the green layer, showing the underlying yellow layer. I also added a subtle drop shadow to the green layer to separate it a bit more from the yellow. 

 Now that the basic texture is done, lets start dirtying it up. I create another layer and set it to multiply. With a very big, soft brush set to a mid brown color I start adding dirt. Concentrate this in nooks and corners where dirt accumulates but also a bit over the whole model for broad dirt. Keep it soft and adjust the intensity by playing with the layer transparency. You can also break up the soft strokes with a grunge texture.

 Then it's time to add wear to the surface. This should mainly be kept along edges but also within large surface areas like the middle of the texture using a broad brush.

This might look good enough but if you've got the time, why not add rust streaks, mud, sootstains and what else your dirty heart desires.
Next thing on the list is to add scratches where all paint has come off to reveal metal beneath. To remain flexible I want to use a Mix Material  in Max for the paintjob and the metal. For this I need a mask which I paint in Photoshop with my Wacom set to brush width sensitivity. With random strokes I paint the scratches which turn out like this.

With all the layers completed, the file should look like this in Photoshop.

Now it's time for the specular and bumpmaps. The reason why I've kept every color and effect in a separate layer in Photoshop is because this makes it easy to derive the spec and bumpmaps from them. You could of course also just reuse the diffuse map but in this and many other cases it would't work: the yellow paint is beneath the green but if you'd use the diffuse as a bumpmap, the yellow, being a lighter color, would be above the green. You also don't want the dirt and wear layers to influence the bumpmap. It also wouldn't work as a specular texture because we want both colors to have about the same shinyness.
So I adjust each layer separatly using brightness/contrast and hue/sat/lightness, hiding the dirt and wear layers for the bumpmap.

By using Actions in Photoshop it's fairly easy to record these settings and apply them to other files which I will do later with the other object.
Here are the resulting maps.

I then create textures for the other part using the same steps as above and use the Action I created to make the bump and speculars.
Now let's get back to Max again and start assigning those maps.

I use the same shinyness map in both Specular Level and Glossyness slots and use Output -> Output Levels (or RGB Levels) in the Texture Map submaterial  to adjust the shine. Since 3ds max4 supports Wiring I wire the Output Levels of the two objects to each other so that any change to one will affect the other. This is a great thing that would have saved me some tears had I been able to do that in earlier projects.

I create a metallic looking material using a bitmap texture, a high Specular Level and a very low Glossyness.

Now each object's material is split into a Mix Material. The painted surface come into one slot, and in the other slot comes a reference of the metal material. This way it's easy to change the metallic look globally. As a mask we use the scratch textures we painted before.

Let's have a look at the objects in good light. It's very helpful to have some basic lighting set up while tweaking materials as opposed to using the standard max 2-light setup. This way you'll better see how the materials react.

Here are some renders of the diffuse color alone, with bump and specular and then with the scratches added. The scratches are subtle but add a neat touch when light makes the metal shine differently beneath the paint. Because if the broad but strong metallic speculars and the dark texture, the metal will be lighter than the paint in light but darker in shadows.