Accurate metalness values for complex materials

[3Danua]

Hello all, I've struggling with complex materials such as plastic painted with metallic paint(with granules of metall), blued steel, parkerized iron, black iron, casted iron, blacked metalls. Overheated metalls. 

I think this kind of materials should have values in between of 0 and 1,but I don't what correct values it would. I try to guess it by eyes, maybe there is something like a chart for this types of materials? 

[3Danua]

anyone?

[davescm]

I've never seen such a chart.
There have been discussions on Polycount but nothing as conclusive as for x use y. With some of those burnt materials lowering metallness along with taking the reflectance in the base colour map down at the same time seems to yield a convincing result. But it seems to be "tweak it till it looks right" rather than a hard science approach, which is understandable as rendering is an, albeit science based, approximation.

For metallic paint you might want to look at an approach using a shader that allows a coating so a layer with a dielectric base paint. A layer with metal particles with variations in height and angle and a layer with a clear coating.

Dave