When do I need a white underbase?

Whenever you print on anything other than white paper or white film. Clear BOPP, silver BOPP, metallized film, brown kraft, colored stock, and dark substrates all need a white underbase layer to make colored inks reproduce accurately. Without it, process inks print transparently and pick up the substrate color.

How do I set up a white underbase in Illustrator?

Create a new spot color named exactly 'White' or 'White Underbase'. Your print provider will confirm the exact name. Set it as a Separation with any screen color (100% Magenta is a common convention). Duplicate your artwork shapes onto a layer below the color art, fill them with the White spot color, and ensure the White swatch is set to knockout (not overprint). Export as PDF with separations intact.

What if my design has transparency effects and needs a white underbase?

The underbase layer has to be solid. Transparency effects behind a white underbase do not work. If you want a color to fade from solid to transparent over a clear substrate, the underbase itself must fade, and the color layer fades in sync. Most RIPs expect a boolean underbase mask, not a gradient. Ask your print provider how their preflight treats underbase transparency before you commit to the design.

What is a White Underbase? White Ink on Clear and Silver Labels

TL;DR

A white underbase is a layer of opaque white ink printed beneath the color artwork to give process inks a solid base to sit on. It's required whenever the substrate is anything other than white paper or white film. Clear BOPP, silver BOPP, metallized film, brown kraft, dark stock. Set it up as a named spot color (usually called White or White Underbase), apply it to a layer beneath your artwork, and make sure it's set to knockout. Never overprint. Overprinted white disappears on press.

Why it exists

Process inks, the C, M, Y, K of CMYK, are transparent. They reproduce color accurately only against a white background. Put a red ink directly on clear film and you see a dim, tinted red against whatever's behind the label. Put the same red ink on silver BOPP and it looks metallic-maroon. The substrate color wins.

The fix is to print opaque white ink first, in the exact shape of the color artwork, then print the process colors on top. The white creates a clean substrate regardless of what's actually underneath, and the colors reproduce as designed.

When a white underbase is required

Clear BOPP labels. The most common case. Any color on clear film without a white underbase is translucent.
Silver BOPP / metallized film. Colors pick up the metallic tint. Used intentionally for shimmer effects, but usually requires an underbase.
Flexible packaging on clear or silver film, pouches, stand-up pouches, flow wrap.
Dark or colored stock, black paper, kraft board, pre-colored substrates.
Window patches, the white-backed portion of a carton that has a clear window elsewhere.

Some brands intentionally skip the underbase to get the metallic or transparent look, but that's a creative choice, not an oversight. It should be specified on the dieline or mentioned in your handoff notes.

How to set up the underbase layer

1. Create a named spot color. In Illustrator: Swatches panel → New Swatch → Color Type: Spot Color. Name it exactly White or the name your printer specifies (sometimes White Underbase, OPWhite, W). The screen color is cosmetic. 100% Magenta or a bright green is a common convention because it's easy to see in the design.

2. Create a dedicated layer. Name it White or Underbase. Place it beneath your artwork layer so the underbase renders first in the stacking order.

3. Duplicate the artwork shapes. Copy every opaque color shape from the artwork layer down to the underbase layer. Fill each with your White spot color. The underbase should cover every pixel where color art lives, but not the bleed or safety zones unless intended.

4. Set knockout, not overprint. Select the white objects, open Window → Attributes, make sure Overprint Fill and Overprint Stroke are OFF. Overprinted white is invisible on press.

5. Export with separations. File → Save As → PDF → Output → Color: do not convert colors. Export with the White spot plate preserved as its own separation.

The white overprint disaster

The most common white-ink failure in packaging: setting the white swatch to overprint. When a white object is set to overprint and sits on top of color art, the RIP interprets it as "add white to the ink underneath", and on a standard 4-color plate set that means nothing is added. The white disappears entirely. The color art prints directly onto the clear or silver substrate as if there were no underbase at all. Thousands of pouches later, you have transparent artwork and a reprint.

Always preview with Overprint Simulation turned on in Acrobat (Print Production → Output Preview → Simulate Overprinting) before you release a file.

Printing on clear or silver film?

Preflight automatically infers when a white underbase is needed based on your substrate and flags missing or overprinted white layers before your file ships.

Check a file →

What is a white underbase?

Why it exists

When a white underbase is required

How to set up the underbase layer

The white overprint disaster

Printing on clear or silver film?