Car dust

                         First open your scene and set rendering method as mental ray. Otherwise you cannot render it clearly because we use mental ray car paint material. Make sure that you have already load mental ray plugin.

                              Switch to polygon mode by clicking F3. Select the parts(bumpers etc) that you want to add dust. Make sure you have correct uv. Go to Create UVs->Planer Mapping option box. Reset the settings. Set Project From as X axis. That depends on your scene. Then click Project. After that go to Window->UV Texture Editor.

                           Click Polygons->UV Snapshot. Set Size X and Size Y to 512(1024 is good). Set Image Format as targa and save it. Go to Photoshop and open it. Create a new empty layer. Switch to Channels tab. Create a new alpha channel with black color. Open the brush tool with white color and paint on the alpha channel. Look at the following image.

                           White color will add more dirt. Grey color will add less dirt. Delete Alpha 1 channel and Background  layer. Your final image with alpha channel will look like this.

                           Save it as a new targa file. Go to Maya and create a new mental ray car paint material. Assign it to car and go to attributes editor. Expand the Dirt Parameters section. Assign file node to Dirt Weight attribute. Open your new targa file which has a alpha channel. Render your scene. You will see gray color dust around the car.

                             You can use brown color lambert material as Dirt Color. Use noise texture as bump map. Set bump depth to 0.01. set noise frequency to higher value(3000).

                            Also you can use your own alpha texture for bump map. Set higher bump depth to looks like dirt. This is my render