An underbase is a layer of white ink printed first, under the color layers, on dark or transparent materials. It blocks the dark substrate and gives color something opaque to print on, so colors appear saturated instead of washed out.
Why underbase is used
CMYK on dark vinyl or clear vinyl is translucent. Without white underneath, red looks pink, blue looks faint, and fine detail is lost. The underbase creates an opaque “white” surface so color reads correctly.
File and print order
The RIP or workflow prints white first, then color on top. File setup must define the underbase area (usually slightly larger than the color to avoid edge show-through). Misalignment or wrong trap causes halos or thin edges.
Checklist for underbase jobs
• White layer or spot defined for underbase area.
• Underbase sized to cover color (with trap if the printer specifies).
• No accidental knockout of white where color should sit on white.
Common mistake
Assuming the printer will “add” white under color automatically. Without a defined underbase in the file, results are inconsistent. Provide a proper white layer or follow the printer's spot setup.