"Applying texture to a flat surface, or flatly applying a texture across a whole image (as a means to age it or degrade it) is simple stuff. Realistically applying a texture to an uneven surface is much more hands-on (more so than simply overlaying a texture and changing the Blending mode)." - James Davies
Here is a link to the written version of the tutorial How to Apply Textures to Uneven Surfaces and the video version is below.
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