Do all discs have a hard-coat covering or something equivalent?
Yes. As mentioned earlier, with Blu-ray Disc the data resides close to the surface. The top of the substrate is 0.1mm beneath the cover layer surface, which means the substrate itself is 1.1mm thick. “A greasy fingerprint could be destructive, since you’re focusing so close to the fingerprint,” says Parsons. As a result, the Blu-ray Disc specification requires that some variation of hard coating be applied to every disc. The spec has a minimum hardness rating that the disc must achieve to survive normal handling by users. A coating is one method of achieving that hardness requirement, and vendors can take different approaches (for example, a spin coating on the disc’s surface).
