But if the “Specular” value determines the IOR for solid materials, I have difficulties imagining a refraction.
Yep, it’s very weird. So IOR actually refers to the Complex Refractive Index and not just the Refractive Index. The Complex part of that refers to also taking into account the Extinction Coefficient, which is negligible in transparent materials but becomes important once light starts getting absorbed. You can read more about that here: https://en.wikipedia.org/wiki/Refractive_index#Dispersion
With the “Clearcoat” it’s different. I can imagine this as a transparent glass or plastic layer with some refraction. But you described it in this video as a second mixed in “Glossy Shader” so that there isn’t any layer thickness and therefore no IOR.
I think I misspoke then – clearcoat is just added on top and not mixed in. Not really physically correct, but it works. You’re right though that it’s not influenced by the specular value or IOR.
