Adobe Illustrator CC - Stroke Overprint problem

Question

Hi, I've found probably a bug in Ai CC (mac) which is quite bothersome.
When you have made a vector, say with for example, with a transparent fill and a random CMYK made color line (with a random thickness) and put it on top of a random image or photo and you are enabling the option "Overprint Stroke" then Ai CC won't let you show/see the overprint preview, even if you have enabled "Overprint preview" at the "Separation Preview"-tab.
It only seems to works when you convert your random selected CMYK color into a Spot-Color.
This is quite frustrating as I use that option quite a lot with the designs that need to be printed in a later stage.
It worked fine in Ai CS6.
Hope someone can help me out, because maybe there's been a new setting that I have missed ?!
Thanks

Answer 1

In Adobe software, "overprint" replaces the rearmost component (color sep) with the frontmost. So in your example, you have a 50% K component replacing a 50% K component. The reason you get what you expect when you change the overprinting object to a spot color is: The spot color is a different component; it has no K component. It prints on an entirely different separation plate.
If you were, for example, to change the overprinting object to 50% C, you would see what you expect with "overprint preview" on, because the result would be 50c50k. And if you made the overprinting object 50c50k, there would be no difference. The total k would still be 50%.
So to get what you want, simply change the overprinting stroke to whatever value of K you wnat it to be. It doesn't need to be set to overprint.
This is, of course, counter-intuitive to what we expect on press, because on a press, an overprint is assumed to occur on a second pass or on a second hit (same pass, different head). In pre-computer days it was quite common to do a "double hit" of K, or a second pass of a solid.
Overprint preview is also often misleading, because Illustrator has no facility for simulating the opacity of inks. It always assumes the translucency of offset process inks. So overprinting a light spot color always darkens even though in reality it often should lighten.
JET