Recently, I've been trying to figure out how to make my painted textures look more detailed and realistic. I've never been able to just use textured brushes or photo overlays and get a decent result out of them. So instead, I decided to look at some close-up photos of things like rust and scratched metal, and really, really study them. And then it hit me.
Those detailed and confusing textures, they are really just shapes. Small shapes, but shapes nonetheless. They have contours, they have lighting on them. You can study and draw the small shapes, much in the same way that you can study and draw the big shapes. All you have to do is to get interested enough in those small shapes to study them in their own right, and spend a good amount of time drawing them carefully.
And when you study real-world textures up close, you'll also notice another thing. There is so much variety! Not one flake of rust is the same as another. They have different shapes, different sizes. Some areas are much cleaner, others are more detailed. Which brings me to another point.
If you uniformly apply the same texture brush, your work will look neither realistic nor good. You will want to think of the textures as a design. Try to vary up the size, the shape and the density of the details. Try to make interesting groupings of a bigger detail, a medium detail, and a smaller detail. Don't give everything even coverage, you'll want some less detailed areas for the eye to rest. Try to work in some bigger color transitions that you can still see when you squint at your image. For instance, you could paint your metals to look brownish near the bottom where it gets dirty, blue near the top where they reflect the sky.
Everything in art is a design, from your overall composition down to the smallest detail. If you stop thinking of textures as something you should be able to quickly slap together with a few textured brush strokes, you can improve your work a lot. Texture has to be earned.
Once you know how to get a good look by painstakingly painting tiny details, you can probably find a more optimized approach using brushes and overlays, as well. But before optimizing your workflow, you have to learn how to create the right look in the first place. Ofcourse, this might sound like it's the hard way to doing art. But I think doing things the hard way is actually the shortcut to leveling up your art.