Hi, @Gnaws. I’m a bit unsure about the exact physics behind it, but the Principled BSDF shader actually uses the specular input differently than older shader models. As someone that used Blender for a long time before the Principled BSDF shader, increasing the specular value of your material usually meant sharper, glossier reflections (or specular highlights using non-raytraced reflections). I’ve heard people say things along the lines of, “Unless you are going for PBR and are going to use the formula provided in the manual as input to the Principled BSDF specular value, you are better off not messing with it from it’s default value.”
I think the value you want to be working with is the Clearcoat input. The manual says that it is an “Extra white specular layer on top of others.” Crank that baby up and see if it gives you the look you are going for. Let us know if you need more help!