Ambient Occlusion Node (Procedural Dirt)

Automatic grime.

The Workflow

1. In the Shader Editor, add an Ambient Occlusion node.
2. Connect it to a ColorRamp.
3. Use a Mix RGB node (set to Multiply).

The Math

AO detects occluded geometry (crevices, corners). The node creates a black-and-white mask where black = deep crevice. By multiplying this with a dirt color, you automatically accumulate grime in the cracks of your model without UV unwrapping or painting a single pixel.