I have not been aware that the dialectric properties have something to do with the reflectance, will have to read up about this, but I gather this has also something to do that metals do not polarise the reflected light, while other surfaces do ... without going into the deeper physics of this, one important point is where on, or rather in, a surface the incoming light is reflected. Many real life 'surfaces' are actually three dimensional structures, albeit thin. Somewhere in this structure the light is reflected and may be bouncing around before leaving again. So, on a microscopic level, not the whole surface may be reflecting, while for metals it is essentially the whole (at least above the atomic level).
Yes, I have been aware of the advances in media/paints to represent metal surfaces. The main point there, if you want ot represent a bare, polished surface, is to make the reflecting metal particles small and with the help fo the paint formulation to coerce them to lie down flat, so that they will form a continuous layer of more or less parallel oriented particles.
For rough metal surfaces, such as the cleaned-up casting for the engine in the picture, this is obviously simpler. The varying angles of the metal particles in the paint mimic the sand-casting surface. I have achieved something similar by rubbing a soft pencil over a black or silver undercoat.
And yes, it is quite amazing what seems to be possible today with products such as the Alclad metal paints, though I have not used them myself yet.