layout transfer to epoxy resin via UV light
For the photoengraving method of transferring circuit artwork, the circuit board cannot just be epoxy clad with bare copper. The copper must be coated with the special photoresist lacquer which undergoes a chemical change when exposed to light so that then later when it is washed in the special developer solution, parts of it will dissolve and parts will stay on the copper, based on the history of light exposure of different areas.

The job of the epoxy substrate is to provide a firm base for the copper traces and electronic components, and to resist damage from all the processing stages.

You can buy board which are already coated with photoresist, or you can apply photoresist to bare copper yourself.