What you need is a multi-layered material Blend. It is this material is VRayBlendMtl . In short, the essence of his work as follows:
The Base material slot - embedded base material. For example, the material mat of wood.
At nine slots Coat materials - invested the right amount of material layers. In your case, you can just put back the same material frosted tree, and then increase the value Refl. glossiness, so that this material was shiny.
As a result, you get a composite material, with frosted tree covered with gloss "paint."
As for exactly thick layer that is physically deposited on the surface of the clearcoat, the thickness of not just one shader can not do. This is possible only through layer geometry simulating lacquer surface. However, it is more than irrational. In almost all cases, the method described by me with a multilayered VRayBlendMtl, more than enough for all situations. I suppose you're going to do the usual furniture. You can safely use the blend material to simulate a layer of varnish😉