I can't speak for anyone else, but I use several methods. For reference, I use the GIMP, but I've also used Photoshop in the past. These ideas work in both.

The simplest is to do a cloud or difference cloud render on a new layer, fiddle with it (posterize, gaussian blur, etc), then set that layer as an overlay blend.

You can also grab a texture from the web (there are tons of websites that have hundreds) and use those for an overlay.

You can also mess around with brushes. Both of the programs mentioned above have brushes that look spotty. You can alter your base color a little, use the brush a few times, and blur or whatnot to get things to blend nicely. You can also use plain old boring brushes but drop the opacity so that each stroke only darkens or brightens things slightly.

The process is honestly a lot of trial and error with your favorite image editing program to get what you want. I keep a notepad file open and write down my steps so that if I get a result I like, I can replicate it.

I've attached a really simple tutorial here for you. Hopefully it's informative. If not, I'll gladly take it in whatever direction you need.

Cheers.

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