How to eliminate CMYK colors behind pantone channel image without erasing whole CMYK layer?

[maxwell]

I am preparing a file for printing that uses CMYK printing with a metallic ink pantone image over the CMYK. The printing agency requires the CMYK background only underneath the pantone design to be removed while keeping visible the rest of the CMYK layers that are not covered by the pantone image. Is there a simple way of doing this?

For example, imagine the file is a blue square. The blue square is CMYK and I have a red pantone circle as its own channel over the blue square. How can I remove the exact area of blue in the CMYK layer behind the pantone circle? In my actual file the "red circle" is a much more complex image. 

I look forward to hearing your thoughts. Thank you!

[Grant H]

Hi, you will have to do it manually, Select the spot channel contract the selection to allow for trapping (IMPORTANT) and then use the selection to mask the underneath layer. You cannot apply attributes to knockout or overprint as in Illustrator or inDesign vector objects. Photoshop will always overprint the spot colour.

/G

[c.pfaffenbichler]

Ideally the printer should provide information on the amount of trapping necessary. 

And the issue can become a little difficult if the spot color areas are not 100% or do not have sharp edges.