After I have pressed the printed copy of my circuit onto the copper plate, there were some obvious errors. Traces not connected, pads incomplete, etc.
I was thinking that if I had a marking pencil/pen of the same composition as what the laser printer had imprinted, I would have been able to repair the board before the etching bath. I tried a permanent marker but it failed as I expected.
If anybody invents something that could do this, they will make a fortune.