What is the difference between overprint and knockout?

Knockout means the top shape cuts a hole in the layer below, like a cookie cutter, so each ink prints in its own area. Overprint means the top shape prints on top of whatever is below, layering ink on ink. Default PDF behavior for most colors is knockout; overprint has to be explicitly set.

Why does my white ink disappear on press?

Because white is accidentally set to overprint. In the PDF preview, overprinting white shows correctly (the white sits on top of colored art), but on press, overprint means the ink beneath the white shows through the white. White ink becomes invisible. Fix: find the white layer in Illustrator or InDesign, open Attributes panel, uncheck Overprint Fill. Or export from a recent Illustrator version where the default behavior has been corrected.

Should black text always overprint?

Yes. 100% K small text should overprint. If it knocks out the color beneath, any registration drift between plates will produce a visible color fringe around the letters. When set to overprint, the text is black ink on top of whatever color is there; a tiny registration shift becomes invisible because the color behind is irrelevant. Most RIPs apply this automatically (overprint black) but always verify in your file.

What is Overprint? Knockouts, White Ink, and Press Traps Explained

TL;DR

Overprint controls how colors stack on press. When overprint is off (the default), the object on top "knocks out" the color beneath. Each ink prints in its own space. When overprint is on, the top object prints over the existing ink layer, stacking color on color. Overprint is essential for small black text and trapping, but a disaster when accidentally applied to white ink or large colored fills. Most on-press print failures trace back to this one setting.

Knockout vs overprint. The default behavior

The default for most PDF objects is knockout. If you place a red circle on top of a blue rectangle, the press prints the blue rectangle with a circle-shaped hole in it, then prints the red circle to fill the hole. Each ink lives in its own area.

Knockout is usually correct. It means the color you see on screen is the color that will print.

Overprint is the opposite. The top object prints over the ink beneath. If that red circle had overprint on, the press would print the full blue rectangle, then print the red circle on top, and where they overlap, the result is a mixture of both inks (on coated paper, probably a dark muddy purple).

When overprint is right

Black text on colored backgrounds. Any body copy set in 100% K should overprint. If the K text knocks out the color beneath, any registration drift between the black plate and the color plates produces a visible fringe. You can see the colored edge peeking around the letters. Overprint black sits on top of the color, so drift becomes invisible. RIPs often apply this automatically (called "overprint black" or "overprint preservation"), but it's safer to set it explicitly in the file.

Trapping. On fast-running flexo and offset presses, adjacent colors that meet at an edge can register imperfectly. Tiny white gaps appear between them. Trapping intentionally overlaps the two colors at the border (the lighter color extends into the darker one) so any misregistration is absorbed. The overlap is implemented using overprint.

Spot varnishes and foil. A gloss spot varnish always overprints the color below, the varnish is a clear coating meant to sit on top of existing artwork, not replace it.

When overprint is wrong, the classic traps

The white ink disappearing act

White ink printed on clear film or dark substrate must knock out. It needs to replace whatever's beneath it with opaque white. If overprint is accidentally on, the press "prints" white on top of the color behind, and because overprinting white effectively adds zero (white pigment over color = color), the white layer becomes invisible on press. The proof PDF shows it correctly because Acrobat's preview simulates overprint. Only the printed result reveals the problem.

Colored fills with overprint left on by mistake. A yellow shape with overprint on, placed over a blue background, becomes green on press. The designer sees yellow on screen, but the press adds yellow on top of the blue and the result is a muddy green. Always unless there's a specific reason for it, colored fills should knock out.

Overprinted 0% fills. Sometimes a shape is filled with 0% of a color (invisible) but overprint is set. The result looks blank on screen but causes a real knockout on press (or no effect at all), depending on the RIP. Either way, unpredictable.

How to check overprint in your file

Illustrator: Select any object, then Window → Attributes. The Attributes panel shows "Overprint Fill" and "Overprint Stroke" checkboxes. If either is checked, that object overprints.

InDesign: Same. Window → Output → Attributes. Overprint Fill/Stroke checkboxes.

Acrobat Pro: View → Tools → Print Production → Output Preview. Turn on "Simulate Overprinting" to see exactly what will print. This is the gold-standard preview. It shows what the press will actually do, not just what the screen displays.

Preflight tools: Automated preflight scans every object and reports which ones overprint. Flags to watch for: "white ink overprinting" (almost always a bug), "non-black ink overprinting" (usually a bug unless intentional trapping), "light color over dark color knockout" (usually correct but worth verifying).

Not sure if your overprint settings will survive the press?

Preflight scans every object for overprint attributes, flags white ink set to overprint, and catches the classic traps before your file hits the RIP.

Check a file →

What is overprint?

Knockout vs overprint. The default behavior

When overprint is right

When overprint is wrong, the classic traps

How to check overprint in your file

Not sure if your overprint settings will survive the press?