Rich black on small text, hairline rules and white objects set to overprint are three of the most common reasons a PDF that looks perfect on screen prints badly, because a normal PDF viewer shows none of them. Small rich black text blurs when the four plates do not register exactly; a hairline or a line under about 0.25 pt can break up or vanish on a high-resolution press; and a white object set to overprint simply disappears. ProofSlip, a Windows app from Automata Labs, checks a print PDF for all three, along with fonts, image resolution, bleed and the rest, and shows each problem boxed on its page so you know exactly what to fix before you send the file.
This guide explains what each problem is, why it happens, how to see it yourself in a PDF, and what to change in the source file.
Check a print PDF for rich black text, hairlines and white overprint, with each problem boxed on its page.
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Why do these problems not show up on screen?
A screen shows a composite: every colour blended into RGB pixels. A press prints separations: one plate per ink, laid down one after another. Problems that live in the relationship between plates, or in how a device renders a very thin line, are invisible in the composite. Viewers such as a browser or a basic PDF reader do not simulate overprint by default either, so an object that will vanish on press still appears on screen. Acrobat Pro’s Output Preview can show separations and simulate overprinting, and that is the manual way to look.
What is rich black, and when is it a problem?
Rich black is black built from black ink plus some cyan, magenta and yellow, used so a large black area looks deep instead of slightly grey. Adobe’s InDesign documentation explains the reason: standard 100% process black may not appear completely opaque, so “to prevent this show through in large black areas or large black type, use a four-color rich black”, and it adds that rich black “is not to be confused with the Registration color” (Adobe: About overprinting).
The problem is the opposite case: small text or fine lines in rich black. Four plates never register perfectly, so a 7 pt caption built from four inks can show coloured fringes and look soft. Body text and fine rules are normally set in 100% black only, which also overprints by default in InDesign (“InDesign automatically overprints 100% black fills and strokes by default”), avoiding white gaps around black type on coloured backgrounds.
Two related traps:
- Registration colour used as black. Registration is 100% of every ink and is meant for printer’s marks. Used for artwork, it puts far more ink on the paper than any press wants.
- RGB black converted to CMYK. Black text from an RGB source often converts to a four-colour mix. It looks identical on screen and turns into rich black text on press.
| Black element | Usual recommendation | Why |
|---|---|---|
| Body text, small captions | 100% K only | No registration fringes; overprints cleanly |
| Thin rules and hairlines | 100% K only | Same reason, and thin four-colour lines register worst |
| Large black areas, big headlines | Rich black, mix per your printer | Deeper black, no show-through |
| Printer’s marks | Registration | That is what it is for, and nothing else |
Ask your printer for their preferred rich black mix and total ink limit; it depends on press and paper.
What counts as a hairline, and why do they disappear?
“Hairline” is the stroke setting in some applications that means “the thinnest line this device can draw.” That is the problem: on a 300 dpi office laser it is visible, and on a high-resolution press it can be almost nothing. Tradeprint’s artwork guide puts it plainly: hairlines “are device dependent, which means they print differently on different printers. They may print well on a 300dpi laser printer then disappear on our 2400dpi press” (Tradeprint: what are hairlines in print).
Printers usually publish a minimum line weight instead. Onlineprinters, for example, asks for a minimum of 0.25 pt (0.09 mm) for positive lines (dark on light) and 0.5 pt (0.18 mm) for negative lines (light on dark) (Onlineprinters: line weight). Your printer’s spec sheet wins over any general figure.
Hairlines hide in a few places: table borders from a spreadsheet, thin lines in placed vector logos, “0 pt” strokes in drawing apps, and fine strokes scaled down when a large graphic was shrunk to fit.
What is white overprint, and why does it make objects vanish?
Overprint tells the press to print an object’s ink on top of whatever is beneath instead of knocking out a hole for it. For process inks, white is not an ink: it is the absence of ink. A white object set to overprint therefore adds nothing on top of the colour beneath, and on press it disappears. Adobe’s Illustrator documentation describes how this happens by accident: a new white object can inherit overprint from a previously selected object, or a coloured object with overprint can be changed to white, and the problem “becomes apparent only when one turns on the overprint preview or printing separations. This causes delays in the production process and reprinting may need to be performed” (Adobe: White Overprint).
Recent Illustrator versions discard white overprint on output by default, but files from older versions, other applications and templates still carry it. The same page notes one exception that matters for some jobs: spot white objects (a real white ink, as used on clear labels or dark stock) are not affected by that option, so if your job uses a white spot ink, overprint settings on it are deliberate and need to be checked against the printer’s instructions.
How do you find these three problems in a PDF yourself?
With Acrobat Pro:
- Open Output preview (All tools > Use print production > Output preview) and turn on Simulate Overprinting. Look for any white elements that disappear.
- Switch the Preview menu to Color Warnings and turn on Rich Black, which Adobe describes as indicating areas that print as process black mixed with colour inks. Or switch to Separations and turn off Cyan, Magenta and Yellow: small text that partly disappears was rich black or converted RGB black.
- Use Preflight with a profile that reports thin lines below a set width, or zoom to 800% and inspect rules and table borders by hand.
Without Acrobat Pro or a similar prepress tool it is hard to check by hand, because most everyday viewers do not simulate overprint or show separations. That is the gap a dedicated checker fills. Our ProofSlip vs Acrobat Pro preflight comparison covers the trade-offs, and the print-ready PDF checklist covers the rest of a file check.
How does ProofSlip check for them?
ProofSlip checks print PDFs on your PC and explains, in plain language, what to fix before you send them.
| What you do | What ProofSlip does |
|---|---|
| Pick a job preset | Sets what “fits this job” means, from business cards to book interiors and paperback covers: size, bleed, spine, safe area, gutter, page count and colour |
| Open or drop the PDF | Checks fonts, image resolution, colour spaces and the plates the file needs, text near the trim or gutter, spine text and the barcode area |
| Read the findings | Reports small text, rich black, hairlines, ink limit, white overprint, transparency and printable comments |
| Select a finding | Shows its page with the problem boxed and the trim and safe area drawn in |
| Export | Writes an HTML report with the same boxed pages; drop a whole job and export every report at once with your name and logo |
ProofSlip also checks EPUB files and Word manuscripts for self-publishing jobs, but for this kind of print problem the PDF check is the one that matters. Files never leave your PC and are never changed: ProofSlip is a check, not a fixer, so corrections happen in the application that made the file.
See every rich black, hairline and white overprint finding on its page, then fix it at the source.
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How do you fix each one at the source?
| Problem | Fix in the source file |
|---|---|
| Small text in rich black | Recolour to 100% K. In Word or an RGB layout app, export with a print preset that keeps black text as K only, if it has one |
| Registration colour in artwork | Replace with 100% K or the printer’s rich black |
| Hairline or 0 pt stroke | Set an explicit weight at or above your printer’s minimum |
| Thin reversed lines | Thicken to the printer’s minimum for negative lines |
| White object set to overprint | Turn off Overprint Fill and Overprint Stroke for that object (Attributes panel in Adobe apps) |
| Spot white with overprint | Confirm with the printer what they expect; do not change it blindly |
After fixing, export the PDF again and re-check it. A fix in the layout file is only a fix once the new PDF shows it.
Frequently asked questions
Is rich black always wrong?
No. Rich black is the right choice for large black areas and large type. It is wrong for small text and thin lines, where four-plate registration shows. Ask your printer for their preferred mix.
What is the minimum line weight for print?
It depends on the printer and process. A common published minimum is 0.25 pt for dark lines on a light background and 0.5 pt for light lines on a dark background, as in the Onlineprinters spec cited above. Use your printer’s figure if they give one.
Why did my white logo vanish in the printed job?
The white object was almost certainly set to overprint. White is not a process ink, so overprinting it adds nothing. Turn overprint off for that object and export again.
Does ProofSlip fix these problems?
No. ProofSlip finds and explains them, with the problem boxed on the page. It never changes your file, so you fix the source and check the new export.
Can I check a whole job at once?
Yes. Drop all the files on the window and export every report at once, with your name and logo on them.
Try ProofSlip
ProofSlip is a one-time purchase in the Microsoft Store with a free trial. Check the last PDF a printer sent back to you and see which findings ProofSlip boxes. More detail is on the ProofSlip product page and the PDF preflight page.
Check your next print PDF during the free trial, before it goes to press.
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Adobe, Acrobat, Illustrator and InDesign are trademarks of Adobe. Microsoft Word is a trademark of the Microsoft group of companies. ProofSlip is not affiliated with Adobe, Tradeprint or Onlineprinters.
Last updated 19 October 2026. Published by Automata Labs, a software company in Canada.