Overlay, dirt with principled BSDF I've got a very clean aluminum metal texture with It's own roughness, metalness, normal etc-texture on a principled shader. It's nice but I want it dirtier.

So I've downloaded some dirt overlays and metal scufs from Poliigon... but I can't seem to find any tutorials on how to use them on the Principled shader. No matter how I try to MIX RGB and such it doesn't look right.

Please help!
Combine your existing Roughness map with your new Dirt/Scuffs map using a Math node with Type = "Maximum". Use the output of the Math node for your Principled shader's roughness socket.