Stop flattening the transparency. Export the PDF with Acrobat 5 or later compatibility.
Thanks so much, Bob.
My printer requires that I use the PDF/X-1a:2001 standard for PDF export, which means that it must be Acrobat 4 or later compatibility.
Also: I don't see how to NOT flatten, when exporting PDF for print; the options for flattening are "low resolution," "medium resolution," or "high resolution"—but there is not an option to NOT flatten. Do you know a work-around for this?
Find another printer.
X/1-a is an archaic format that has no support for transparency, color management or RGB images.
Short of that, you’ll have to live with it. Those fine lines are called stitching and shouldn’t show up in print.
I just tried a "hack" by exporting from InDesign as "Acrobat 5 and later" compatible (NO lines, by the way—thanks Bob!), then opening that PDF in Acrobat and saving it as a "Print Ready" version (PDF/X).
The lines came back...
Thanks again, Bob.
I'm going to check with my printer to verify that the stitching doesn't show up in printing. Unfortunately, I don't have the luxury of choosing another printer at this point.
I'll come back and report what they say, for curiosity's sake at least.
Of course they came back. This isn’t magic. The only way to flatten transparency is chop it up into pieces.
I wasn't thinking it was some "magical" reappearance. I was simply hoping it was an artifact that was the result of the initial export, not something inherent to the layers themselves.
I guess, because I don't understand why they are there in the first place, I didn't understand that they would return simply by changing the output compatibility.