What kind of PC board coating is this?
The boards appear to be photoresist boards, i.e. bare PCBs factory coated with photoresist material, then protected by a thin sheet of plastic. Here is an example of such PCBs.

Over time, even if such boards are stored in a relatively dark place, the photoresist gets exposed and becomes unusable. It just needs to be scrubbed off for the bare PCB to be used like any other copper PCB.