Polyester (PET) films turn up in many forms in large format work: clear and translucent display films, white opaque films for durable posters, technical drawing films and some backlit stocks. They share a few traits that matter when you are thinking about a coating. They are dimensionally stable, they do not tear easily, and their surfaces are very smooth. Most printable versions have an inkjet receiver coating on one side, matched to a particular ink type.
The receiver coating is what you are coating
On an aqueous or pigment film, the ink sits in a receptive layer rather than on the polyester itself. That layer is the part a solvent coating actually meets, and receiver chemistries differ between manufacturers. Some take a solvent overcoat well; others may cloud, soften or lose adhesion. On films made for eco-solvent, latex or UV inks, the surface treatment is different again. Do not assume two clear films behave the same because they look identical.
Clarity and backlighting
With clear and translucent films, the appearance of light passing through the print is often the whole point. A coating that looks perfect under room lighting may show streaks, roller marks or slight haze on a lightbox. Check coated samples in the actual viewing conditions. Our page on backlit films covers lit display in more depth.
Practical checklist
- Identify which side carries the receiver coating, and coat only the printed side unless you have tested otherwise.
- Make sure the ink is fully dry. Smooth films slow evaporation from solvent-based inks; see drying before coating.
- Watch for static. PET films attract dust readily, and any particle will be sealed into the coat. Wipe down and coat in a clean space.
- Check the coated film still lies flat. A coating can introduce slight curl on thin films.
- Test adhesion on solid colour and on bare receiver coating, as either can be the weak point.
When film lamination may suit better
Because PET films are smooth and flat, they are also easy to film-laminate without the trapped-air problems seen on textured media. For some jobs that is the simpler route. Where liquid coating comes into its own is when you need to protect large runs quickly, avoid an extra layer of film, or keep a particular finish. Liquid vs film lamination compares the two.
If you are trialling a new film, a coated and uncoated strip side by side tells you most of what you need. Send us your printed film, or talk it over with us on 01252 624 411.