Unable to reproduce results with Principled BSDF

Now why would you want to do that?

And in close-up:

– Rendertime went from about 4 minutes to around 6 minutes.

– It looks horrible to mix two Principled Shaders. And: no, you can’t use only one Principled there, because:

    1. The Principled Shader has Fresnel built in and you have no control over the roughness of the Glossy and also the Roughness of the Diffuse should not be dependent on the Glossy Roughness. (At least not in this shader.)

    2. The Glossy Node has a different Color than the Diffuse Node!

Well, if this is to learn, then great!

Practically, when you want fast and easy PBR, then the Principled Shader is often the way to go, but you lose a bit of control..

Just for educational  fun, here are a few renders; from left to right: your Node setup, Kent’s Node setup and finally mine:

You seem to lose some definition (shape information) in the dark areas, they become a bit ‘muddy’. Overall it looks a bit more flat (less highlights in the orange and green).

But still a great effort! (You did forget to connect the Normal Map to the Diffuse Shader…I connected that in this render..)

 the diffuse shader for the grime did not have a normal map,

I think it did:

But indeed your render looks a lot better…. I probably didn’t get the Colorramp after the Fresnel Node the same as yours, these things can be quite sensitive;)