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I've tried adjusting the channel mixer of the images to C:40 M:40 Y:40 K:100 (as recommended)
By @Dean266530218t9u
I don't know who recommended this, but it's not the proper way to do it. The proper way is to get the correct CMYK profile for the actual print process. A CMYK profile characterizes an actual print process, defined as an offset press calibrated to a certain standard, using certain inks on certain paper stock. There is no generic "CMYK". CMYK always refers to a physical print process, with the physical limitations inherent in that process.
There is a limit to how much ink the paper can absorb. This ink limit, known as TAC (Total Area Coverage), is built into every CMYK profile. It is different in every profile. This limits and defines the black level. In offset print the black level is fairly high, much higher than for inkjet printers.
If you convert from RGB 0-0-0 to your target CMYK profile, you can see what the deepest possible black is. In ISO Coated v2 (ECI) 300%, for instance, commonly used in Europe, the deepest possible black is 78-68-58-94. You cannot get any deeper blacks than that. In the Photoshop default US Web Coated (SWOP) v2 (which may not be appripriate in your case!), those numbers are 75-68-67-90.
If you exceed TAC, you get ink smearing and drying problems.
Get the correct profile, and work with the deepest black in that profile.
On screen, this will not be correctly represented unless you adjust your calibration parameters to a visually matching black point. This will be much lighter than the monitor's native black level. In most cases, this will correspond to a monitor black level around 0.8-1.2 cd/m². Proofing to the CMYK profile will reflect the CMYK ink limit, but not the physical paper reflectance, which lifts the visual black further.
Thank you for your detailed response.
I will definitely look into what the exact CMYK profile needed is.
Yes, this would be better than guessing, however for the black background content, it doesn't really matter as long as it is 220% total ink coverage with 100% K.
It is important for the continuous tone photographic content through.
Do you have any recommendations in terms of exporting the image out of Photoshop without this grey wash I mention?
Thanks again!
By @Dean266530218t9u
I believe that this is just a preview of lesser ink density. 400% ink has more density than, 300% or 220% etc.
Can you provide a lower-resolution flattened RGB duped file for testing?
It is very common for print service providers to ask for very large areas of rich black (often artificial panels) to be limited in total ink at a much lower value than the four colour black found in photographic images which don't usually have large solid areas (yes, there are photographic exceptions).
Anyway, they have quoted 220% total ink. This should be more critical than hitting 40cmy100k.
100k is important though as it will add more density than the other colours.
All that being said, you do need to know the press condition to convert to, regardless of any post separation editing to hit specific CMYK values.
For example, the following RGB black conversion to CMYK is for the Fogra39/ISO Coated v2 condition, using a profile created with a 220% ink limit:
@Dean266530218t9u You've received some great advice here - I'd add - Make a duplicate, save the original, now -working on the duplicate, flatten any layers before saving, I'd recommend using "save as" rather than 'export'. Mak sure the CMYK ICC profile is embedded
You'll want to check Enable legacy "Save As" in file handling Preferences
I hope this helps
neil barstow, colourmanagement net - adobe forum volunteer - co-author: 'getting colour right'
google me "neil barstow colourmanagement" for lots of free articles on colour management
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