White ink opens up work that was not possible on CMYK alone: graphics on clear and coloured media, window and backlit pieces, prints on dark or metallic substrates, and spot-white highlights. Most white-capable printers use eco-solvent, latex or UV-curable inks, so the general guidance on those pages applies. What changes with white is the structure of the print, and that is what affects coating.
Thicker, layered prints
White is usually laid down heavily to get enough opacity, and is often printed in layers: white under colour, colour then white for second-surface viewing, or colour-white-colour sandwiches for day and night graphics. That leaves more ink, in more layers, than a standard print. In practice this means:
- Longer drying for solvent-based inks. Heavy white areas hold more solvent and need more time before coating. See drying before coating.
- Intercoat adhesion matters. If the bond between white and colour layers is marginal, a coating that slightly softens the top layer can expose it.
- Visible edges. Where white stops, there is a step in the surface. A coating can pool against it or show a line.
- Uneven sheen. White areas may take a coating differently from bare media alongside them.
Which side are you coating?
For first-surface prints, the coating goes over the ink and protects it in the usual way. For reverse-printed, second-surface graphics on clear media, the viewing face is the clean film and the ink is on the back. Coating the ink side can still be useful to protect it from scratching during handling and installation, but it must not interfere with any adhesive or mounting. For optically clear applications, test that the coating does not affect clarity where light passes through.
Test before you quote
- Print a test that includes white-only areas, full layered stacks and bare media.
- Coat it the way you would in production and look closely at the white edges.
- Check adhesion over the white layer specifically, as that is where weaknesses usually show.
- For backlit pieces, view the coated sample on a lightbox and compare it with an uncoated one.
If the job is a lit display, our page on backlit films covers further points. For more on the method itself, see coating defects, which covers pooling and edge lines in more detail.
White ink jobs vary a great deal from one printer and media to the next. If you would like us to look at a layered sample, post it to us or call 01252 624 411 to discuss it first.