How do you plan to control melting of the paint where you do not want melting? Seems to me the heat could migrate through the copper.No, the board will get painted black as it absorbs the most energy. After that paint will get burned off with the laser exposing the copper to be etched away. Using laser should make using very thin traces possible because lasers have very small focal point. Painting the board black allows the use of weak lasers ,generally in range of 200-500mW. And those lasers are very cheap mostly in range of 2-3$.