print design - InDesign – Overprinting white at a Guangzhou printshop

I'm just about to send a CD-booklet 134mmX122mm (34pp)for print in a Guangzhou printshop – and the language barrier makes it a bit difficult to get all points across; I'm wondering about what I need to explain about the overprinting. Every page of the book is slightly yellow-toned (A-master yellow-toned) with white background, greyscale illustrations on every page.

When overprint preview is on, greyscale illustrations blend with the yellow pages like they're supposed to, removing the white background of the illustrations.

How can I make sure that overprinting behaves in this way when they print the booklet? The obvious solution, I guess, would be to print on yellow toned stock paper, but since there are full-color images on some pages – that won't be possible. How can I make sure that the printers overprint the white correctly? Does anyone have any experience with this?

Answer

The way you'll make sure overprinting is applied is simply by adding your background color to the color recipe of your gray illustration.

Background CMYK + Gray CMYK = Fake Overprint of the gray.

If the gray is a rich gray, you don't need to literally add the ink; as long the gray contains the same value or more of yellow/cyan/magenta than your background, you'll be fine.

And then also add the overprint setting but at this point it won't be necessary!

You're right to do something more "forceful" than trying to explain yourself. It's very hard to deal with printers in China and you need to provide your files as easy to print as possible. They also have old RIP and your overprint settings could be ignored or simply removed.

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In Adobe Acrobat Preview, you can verify your results or any software that shows you the CMYK separation

That's how it should look like. It will give you the same result as the gray overprint since the background and the gray add up together.

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There's no white "knockout" on the yellow, that's like overprint: Every separation of cyan, magenta and yellow contains the same amount of ink as the yellow background.

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That's what you don't want. That's what could happen if you don't enrich your gray and only use an overprint trapping on your gray separation.

The method above will guarantee you the other separations won't get knockout or trapped without overprint like this!

If you're using that overprint effect to create 2 shades of black when the illustration overlaps the yellow background, you'll need to do some Pathfinder work on that illustration to split it, and then apply the 2 types of gray where you want them (eg. the rich gray with yellow on the background part, the gray without yellow on the white part, etc.)