Physically correct render of material covered with another material What is the physically correct way to setup light shaders for such object as wood plate covered with varnish? Wood and varnish have their own indexes of refraction, however they are kind of a single material now, since you can not split them apart. What is the common practice for such kind of things in Blender?
You can layer reflections by using multiple layered mix nodes. This way you can controll roughnes, ior and bumpmap of each reflection layer separately. For example this is a simple carpaint shader using this technique.

it's far from perfect - it's just for demonstration.