Step-by-step: Make the most of your layer masks
Start by drawing the object you want to scuff up. In this case it’s a suit of armour. Notice how the armour is constructed out of different metal elements that can be masked separately, so they can quickly be modified later on. When painting objects like these, try to understand how they’re shaped. It helps you to paint on the dirt more logically.
For each material, I create a brightly coloured layer on top so they can be masked out separately. Using bright colours helps to see where your mask will have gaps. So if you can still see the underlying layers, go over them a second time. Now go to the Layers panel and Ctrlclick the thumbnail of one of the coloured layers to select its content.
Create a new layer and click Add Layer Mask at the foot of the Layers panel. Your selection becomes that layer’s mask. Now fill the layer with a dark brown colour, click the Channels tab, hide the RGB channels and unhide your new layer mask. Using black textured brushes, paint over the white areas to clean up the armour and generate an organic dirt map.