Printing PCB layouts with Laser Printers may prove to be very difficult because of their inherent printing quality. Inkjets smudge ink and have less blank spaces in their solid prints, whereas laser prints have blank dots if inspected with a microscope (or milliscope). I just made up milliscope – hahaha.
Seriously, take a look at the following 1200 Dots Per Inch (DPI) laser print on a translucent paper:
It is doomed for failure. All those white spots will leave the conductive copper tracks unprotected.
Some methods that proved to be effective are:
1) Print the layer twice and align them to take advantage of randomness of the translucent paper surface and laser printer.
2) Print the layer un-mirrored and the other mirrored. Then align them. This is the method I have tried, and you can see the results here: