Problems with mixing two principled shaders

Hello all. I am trying to make a heavily rusted, painted metal material by mixing two principled shaders. Instead of nice rusty spots on the paint, I get big black artifacts, see the picture.



This does not happen when I try the same with two diffuse shaders instead of the principled shaders. I wonder if I am doing something wrong, or is it not possible to mix two principled shaders. Simplified demo file attached. http://pasteall.org/blend/index.php?id=48535

Looking at the node setup, your test scene does exactly what I’d expect it to. Perhaps if you could post an example of what you hope to achieve.