from what I can see on the pictures, there seems to be a layer of carbon on the top, plus they say that the boards are 6 or 7 plys so I am thinking that there is fiber reinforcement inside, roadside and topside too.
In that case, I wouldn’t keep sanding to get rid of the layer, maybe just uniformly scratch the surface so the paint adheres better and paint than clear coat. @brown5tick probably knows more about this kind of stuff tho.