Fig 1. Clean and Dirty

Often realism in computer graphics can be greatly enhanced by adding imperfections to color textures. These naturally occurring features, a result of an object's exposure to the physical environment, can include rain staining, scratches, scarring, bleaching, corrosion and the accumulation of dirt.

Fig 2. Dirt

The Dirt tool (see Fig 2) has three main parameters that control the dirt calculation process:

Radius 

One way to think of this is to cover your object completely with dirt, then get a felt covered ball of the defined Radius and rub it over the surface. If the ball is large, large areas of the polymesh will stay dirty because the felt won't be able to reach them. But if the ball is small, then only narrow crevices will retain dirt.

As the Radius increases, so too does the calculation time.

Grain 

This controls the accuracy vs. speed of the dirt calculations. As you increase the Grain percentage, the speed increases but the accuracy drops. Reasonable values here are between 5% and 50%.

If the Grain is very low there's a chance that the whole object gets colored black. This is a bug; increase the Grain until the problem disappears.

Smooth 

Increase the Smooth amount to blur the final result.

Fig 3. Radius vs Grain

To calculate the dirt, click on the Calc button. It will turn into an Abort button, which you can use to stop the calculation before it would normally finish.