On pre-etch boards. I read some time ago that PCB board manufacturers oxidize the copper before adhering it to the fiberglass substrate. Bare copper is difficult to bond to, but a thin oxide layer bonds well. I use cheap board that often have defects and I clean them with 400 grit sand paper on an orbital sander. Then I clean the board with lacquer thinner. What ever is in lacquer thinner creates a thin oxide layer, and aid in adhering the toner to the board. Found this also allows a lower laminator temperature, and less distortion of the toner.
Think this pre-etch may also be applicable to the acetone transfer method that has been documented here recently.
RD