I would bet, especially in the publishing industry, that overprint fill causes more print errors and headaches than any other print function. Here is an explanation of what overprint fill does and how to avoid possible problems with it. The tricky thing is that it seems that Illlustrator applied this attribute to elements randomly. So an element you never applied this to may suddenly show up with this attribute applied.

Here is what overprint fill does in the simplest terms I can describe. When overprint is applied, instead of knocking out elements behind the foreground elements, it layed one ink over another. Which really isn't that bad if that is what you want—it almost looks like a transparency type effect.

Here are three circles — magenta, cyan and yellow. The first image overprint fill is turned off. The second image overprint fill is turned on:

But when applied to a foreground element (the topmost element) that is lighter (say yellow) than the background element (black)  you will end up with unexpected results— probably a black that may have a yellow tinge to it. Or possibly the yellow may not show up at all if the black is a rich black (100 percent CMYK). It depends on the ink, the yellow may show up fine depending on the order of the plates.

Here is a yellow foreground element on a black background element without overprint fill applied:

This is that same image with overprint fill applied —viewing using illustrator's overprint preview option on.

The yellow dropped out. This could be a big bummer if that yellow element was important.

Without overprint fill selected the foreground element, will knock out—or remove— the elements behind it. So if you have a light foreground element and a dark background element, chances are you do not want overprint fill applied to the foreground.

This is what the the black plate would look like with overprint fill turned off. The area where the yellow text is on top, is knocked out.

A classic mistake is when a white foreground element is layed on top of a black background element and overprint fill is applied to the white foreground element. It will look just fine in the pdf file (unless you have the overprint preview selected) but when it is separated in your  printer's rip, the white will disappear.

The simple thing to remember is to never apply overprint fill to a foreground element that is lither than the background. There may be some exception to this rule, but this is a general rule of thumb.

If you are working in illustrator you should make sure you have the overprint preview option on by selecting it under the view window (see illustration).

To remove overprint fill from an element in Illustrator, open it (I am working in CS3), and under the windows menu select attributes (see illustration).

Select the elements you want to check and look in the attributes panel. If the overprint fill option is checked, it is on. Simply uncheck the box to turn it off. (See illustration below)

If you are using a pdf workflow, it is a good idea to select the overprint preview option (see below) in acrobat. With overprint preview selected, you will see if an has overprint fill applied to it. This can save you countless errors!

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