show-through

Show-through occurs when ink, dye, or coating applied to one face of a substrate becomes visible on the reverse side. The printed image appears as a faint shadow or ghost outline when you look at the unprinted surface, or it may show clearly through thin material. This is distinct from strike-through, where ink saturates completely through the substrate; show-through is a visibility problem even when the material itself remains intact.

The root causes are substrate weight, absorbency, and ink formulation. Lightweight papers below 60 gsm are prone to show-through because light passes through them easily and the ink layer sits on or near the surface rather than being fully absorbed. Uncoated papers with high porosity absorb ink unevenly, creating dark patches that transmit light. High ink saturation, particularly with dense solid coverage or heavy pigment loads, makes show-through worse. Conversely, premium coated stocks with proper sizing and sealers, typically 80 gsm and above, minimize the problem.

The term takes its literal meaning from the optical phenomenon: printed matter literally shows through. In offset lithography and flexography, show-through is managed by controlling ink film thickness, reducing dot gain, and selecting paper with appropriate opacity ratings. Opacity is a formal measure: most commercial printing papers rate between 80 and 94 percent opacity, meaning that percentage of light is blocked. A 90 percent opacity paper still allows 10 percent transmission, enough for dark solids to ghost through thin sheets.

Show-through is most critical in publications where both sides carry important information: newspapers, magazines, direct mail, and double-sided instruction cards. Readers find reversed text or images distracting or unprofessional. In packaging and labels, show-through can obscure brand messaging or regulatory information. The problem worsens when printed sheets are backlit or held to a light source, such as in window envelopes or translucent applications.

Printers address show-through through substrate selection first: upgrading from 50 gsm to 80 gsm newsprint, or specifying text-weight instead of bond for forms. Ink recipe adjustment follows: reducing pigment concentration, increasing binder proportion, or switching to lower-density inks. Wet trap sequences matter in multicolor work, since fresh wet ink sits above dry ink and shows through more readily. Some operations use calendering to compress the paper surface and reduce optical transmission, though this affects other properties like brightness and feel.