@The_rex
Ususally I paint overlays myself.
Just some crazy white spraycan lines on solid black-smudging, sharpening highlighting , smoothing and so on till i got something that looks like this.
This is just a small sample...i usually make these textures 512x512
If i want fabric new metal i make it more dense and keep it a evenly grey...if i need used metal, more stronger differences in the dark and light grey.
After that i play with translucy and slight colorvariations.