When doing duplex printing on a LaserWriter II, many users find a smudge on
the top of the second page. It is important to understand why the smudge
appears. Most people do duplex printing the following way:
They fill up the paper tray and print out a number of sheets (for example,
50 pages). They then take those 50 sheets from the output tray and put then
back into the paper tray with the blank side up. Then, they print the
second side on the pages they just put back in the tray. Presto! Duplex
two-sided pages.
The problem with printing this way is the pressure that the paper-pickup
roller puts on the paper, as it is pulled out of the tray. When printing
side two, side one is face down in the paper tray. As each piece of paper
is pulled out of the tray, the pickup roller applies pressure to help
separate the pieces of paper. As the piece of paper is pulled out of
the tray, toner from side one of the page is smeared onto the top of the
next piece of paper. The smear is the caused by the pressure of the
paper-pickup roller as it pushes down on the pieces of paper.
There is no way to prevent this from happening. That is why Apple states in
the LaserWriter II owner's manual that duplex printing should be done
manually, one sheet at a time.