once you done with the brushed metal, assign a scratches texture map to specular roughness through blend colors.
This is a technique Lorne Kwechansky shared on the mailing list somewhere last year, adapted for the standard_surface shader (his was for alsurface, RIP). In my opinion this is the best approach I've seen so far, using the anisotropy instead of relying on high frequency bump maps which need very high AA levels to clear up.
You can push this much further for metals (increase the outputmax value), this is just for a latex material I was working on.
The scratch map he created by scattering curves in Houdini can be found here: https://www.dropbox.com/s/sim6ts2935dj8fj/scratch_test_v010.exr?dl=0
Thank you for the answers, everyone.
@Zeno Pelgrims that's exactly the result I'm looking for. I already tried the high frequency bump map route but like you said the AA was getting hefty. I'd really like to see what Lorne Kwechansky came up with. (But the dropbox link doesn't seem to work. Could be permissions?)
Just one question, I assume the +1 and *0.25 is to normalise the specular rotation?
Also, great work on POTA 😉
Directly quoting Lorne here, but note that it is for alsurface which has different ranges for the anisotropy parameters.
"Shader time...I'll go full details here since you guys might want to try this out for yourselves.
As for the image, here's a new link, looks like the one he posted expired: https://www.dropbox.com/s/sim6ts2935dj8fj/scratch_test_v010.exr?dl=0
@Lee Griggs Could you please give me an advice on the above mentioned subject. I want to make a car paint material with micro scratches on it. In real life these scratches appears on the top layer, on the coat. I assume that I cant use standard car paint shader for that purpose, because its coat layer doesnt have specular anisotropy/rotation parameters. I imagine that a solution could be a layered material where material #1 will be my base and material #2 will be a coat simulation (with base weight =0 and specular parameters based on yours scratched OSL).
P.S. I am sorry if it a complete nonsense or an obvious problem, I am two day new to the C4DtoA