Underbase, overprint and white-only artwork are different intentions.
A white underbase sits beneath CMYK or spot-colour artwork to reduce the influence of a dark, coloured, metallic or transparent material. White over coloured artwork can be used in other layered constructions, and white may also print alone as text, a pattern or an image. The order matters because inks are not interchangeable layers of opaque paint. The selected process, viewing side and material decide which sequence is workable.
Describe where the finished item will be viewed. Clear material may be read from the printed face or through the reverse. A label may need an opaque information panel while leaving a window around the product. A metallic board may use white selectively so some colour appears conventional and other areas retain the metal effect. Mark these intentions on a composite visual before the separation is built.
White ink needs a real spot separation.
The ordinary Paper or white swatch in a layout application usually means no ink. It cannot identify a white-ink plate. White production data is commonly built with a supplier-specified spot-colour name, displayed on screen as a conspicuous colour so the designer can see it. The RIP or prepress workflow maps that named separation to white ink.
Follow the exact swatch spelling, colour type, density and file arrangement requested for the job. A generic name such as “White” may not be recognised automatically by every workflow. Keep the white objects on a clear layer and avoid converting the spot to process during export. The same separation principle applies to the coating and foil masks described in the special-finish artwork requirements.
Overlap settings keep CMYK and white together.
If coloured artwork knocks a hole out of the white plate, the material can show through precisely where the colour needed support. If white overprints an area intended to remain clear, the window can disappear. These are not purely visual layer mistakes; they change which separations output in the overlap. Overprint attributes and object construction must follow the white-ink workflow supplied for the job, while the production process determines the physical ink sequence.
Use the source vectors to build the relationship. When a white plate needs to follow a complex image silhouette, ask whether prepress will generate it or whether a supplied mask is required. Automatic generation may cover every non-transparent pixel, while the design may call for selective white. Inspect the finished PDF plate by plate using the overprint and knockout checks.
Small offsets around the underbase are job-specific.
A white edge visible outside a coloured object can look like a halo. Reducing the white mask slightly may help in some workflows; expanding or trapping it may be needed in others. The suitable approach depends on registration, substrate, printing sequence, object size and whether the white is meant to be seen. Do not apply a remembered choke value across an entire file.
Flag narrow lettering, fine reverse gaps, small codes, transparent windows and shapes close to a cut or fold. Request the minimum detail and any offset instruction for the chosen process. When packaging structure is involved, align the white plate to the approved artwork and packaging dieline, not to an unscaled visual mock-up.
Opacity and colour need a material sample.
White ink does not automatically behave like white paper. Its opacity, surface and colour can vary with the ink system, number of passes or layers, material and viewing light. CMYK printed over it may still differ from the same values on a conventional white stock. A screen proof can confirm position and separation logic but cannot demonstrate the physical opacity of the finished construction.
Use a sample that represents the proposed material and ink sequence when show-through, product visibility, brand colour or small-text contrast matters. View clear work against the backgrounds it will encounter, not only on a white desk. Record the reference with the brand colour details, then use the proof comparison to state which physical question the sample must answer.