Can metallic car paint be exported in GLTF? I'm trying to export a car model with red metallic paint as a GLTF model. A diffused red metallic with a high gloss clear. It appears that with GLTF it's either one or the other? 

I can get somewhat of a metallic paint look by sharpening up the roughness of the metallic, but then it looks more like anodized metal than car paint. 

Is this a limitation of the GLTF format or does anyone know of a workaround? Thanks.
(Answer updated 2020-05-13)

You're looking for a feature called clear coat.  There are three relevant inputs on the Principled BSDF shader node, called "Clearcoat," "Clearcoat Roughness," and "Clearcoat Normal."

These three settings will export to glTF files beginning with Blender version 2.83 (still in Beta as I write this).  It will import from glTF files beginning in Blender 2.90 (currently in Alpha).

Clear coat is not part of the glTF 2.0 core specification, but it was added in a glTF extension called KHR_materials_clearcoat.  Not all glTF viewers can display clear coat yet, but some of the major ones have released stable versions that can, including ThreeJS, BabylonJS, and Filament.  The gltf-test project has a test of clear coat with a summary of some frameworks that support it.