Erindale Woodford shows how to apply weathering with shader nodes in Blender.

Check out this tutorial from Erindale Woodford, which shows some layering strategies for creating weathering effects using Blender shader nodes. The tutorial covers a few examples from mud, rust, dust, and scratches applied along with different materials.

Woodford covers many standard techniques for creating weathering effects like edge detection, map creation, and masking.