A matte car paint can be created with only five nodes:
- Principled BSDF
- Velvet BSDF or another Principled BSDF
- Layer Weight
- Mix Shader
- Material Output
Material using Velvet BSDF for sheen
The BMW demo scene was create by Mike Pan

The Principled BSDF should have a Roughness between 0.3 and 0.5 depending on how matte the paint should be. I left all other parameters at their default, but the Specular value could be adjusted as well.
The Velvet BSDF in combination with the Layer Weight and Mix Shader nodes are the key to create the typical sheen of the matte car paint. The Facing output of the Layer Weight is used to blend between the Principled BSDF and Velvet BSDF.
Material using Principled BSDF for sheen

The BMW demo scene was create by Mike Pan

In this variation the Velvet BSDF has been replaced by a Principled BSDF. The Specular parameter can be adjusted to control the amount of specular reflection in the areas with the sheen effect.
Specular set to zero for Principled BSDF with sheen