This attribute is used to coat the material. It acts as a clear-coat layer on top of all other shading effects. The coating is always reflective (with the given roughness) and is assumed to be dielectric. Examples would be the clear-coat layer for car paint or the sheen layer for a skin material. For example, for an extra oily layer or wet skin. Other examples would be objects that have been laminated or a protective film over an aluminum cell phone.
The Coat layer simulates a dielectric (think plastic, glass, resin/enamel, many liquids) which reflect light without tinting (save maybe some very minor polarization effects), while metals, on the other hand, tend to filter the color of whatever it is they are reflecting, even at grazing angles. Therefore, if what you want to render a bare metal, the Coat Weight should be 0.
(Coat Color: Gold)
Coat can be used for materials such as metallic car paint, the reflective coating on a balloon or a bubble for example.
When combining a Coat (low roughness) on top of a Specular (high roughness), the sharp Coat disappears at the center due to Fresnel.
This is the color of the coating layer's transparency.
In typical real-world cases, the Coat Color should always be white and is there mostly for artistic control.
In the example below, the red Coat Color acts as a clear coat layer, tinting the specular reflection underneath.
In the example below, a blue and white checker texture has been applied to the Base Color and the Coat Color has been changed to yellow. When combined, the yellow clear-coat layer on top of the base color gives a green tint similar to a semi-transparent layer of paint or lacquer.
You can also define how the coating affects the underlying Base Color by connecting a texture map to the Coat Weight.
Coat Color can also be used to layer a stencil texture. In the example below, a type image has been connected to the Coat Color, and the Coat Roughness has been increased. Notice how it appears unaffected by the specular highlight that appears to 'sit' underneath it.
Rollover image (without Coating)
Textures connected to Coat Color
Controls the glossiness of the specular reflections. The lower the value, the sharper the reflection. In the limit, a value of 0 will give you a perfectly sharp mirror reflection, while 1.0 will create reflections that are close to a diffuse reflection. You should connect a map here to get variation in the coat highlight.
The IOR parameter (Index of Refraction) defines the material's Fresnel reflectivity and is by default the angular function used. Effectively the IOR will define the balance between reflections on surfaces facing the viewer and on surface edges. You can see the reflection intensity remains unchanged, but the reflection intensity on the front side changes a lot.
The Coat Normal affects the Fresnel blending of the coat over the base, so depending on the normal, the base will be more or less visible from particular angles. Uses for Coat Normal could be a bumpy coat layer over a smoother base. This could include a rain effect, a carbon fiber shader or a car paint shader where you could use different normals (using e.g. flakes) for the coat layer and base layers.
Coat Normal is suited to situations where the layer may be uneven, for example, an oily or wet layer, rain on streets or glazing on food.
Rollover image (left). Puddles are created with a glossy texture map connected to Coat Normal via bump shader (right).
When you specify a Coat Normal, it affects just the coat, and none of the layers below it (diffuse, specular, transmission). In the example below right, the rock material appears to have a clear coat layer, when a bump texture is connected to the Coat Normal.
File and noise textures connected to Coat Normal