As previously noted, it is highly recommended for all aqueous inkjet-printed canvas output to be coated. This will help ensure crack-free, consistent stretching, particularly for ‘gallery wrap’ applications. There are exceptions, such as short-term applications (including temporary decorative pieces). And when using a solvent or latex printer, coating is not necessary unless the client specifically wants to achieve the appearance of coated canvas.
Coatings are typically available in either gloss or satin (lustre) finishes. Gloss coatings are often chosen to provide additional protection for the canvas or to bring out the density of the blacks and the vibrancy of the colours in the graphic, while satin or lustre coatings may be preferable for a more subdued appearance or to reduce glare from high levels of lighting.
There are both spray and liquid coatings available. It is important to choose products that have been specially formulated for and tested with inkjet-printed fine art and photography. The long-term effects of other ‘off-the-shelf’ coatings on canvas graphics are unknown.
Liquid coatings can be applied by automated machines, which are available in a variety of widths and are designed to operate in a controlled environment, with consistent temperature and humidity and free of dust and dirt. Other liquid coatings come in cans; these may be brushed or rolled onto the canvas or applied with a high-volume, low-pressure (HVLP) spray gun, which also requires a well-ventilated and dust-free environment.
Compared to other sprayers, an HVLP spray gun provides more control over the application process and wastes less of the liquid coating in overspray. Some sign shops do not have sufficient space for spraying, so they opt for coatings that have been formulated to work equally well when applied by brush or foam roller. (That said, it may be possible to set up a spray booth in a limited space.)
When rolling liquid coatings onto canvas, these steps should be followed:
- Make sure the ink is dry. A good rule of thumb is to wait 24 hours, but to test whether or not a print is ready for coating, lay the canvas on a flat surface and cover it with butcher paper. The evaporating glycols will cause ripples to appear in the butcher paper, which should be replaced periodically when it is wavy. If no waves appear in a fresh sheet after a few hours, then the print is dry enough to coat.
- Start with a high-density white foam roller and a tray typically used for holding paints; both are available in the paint department of any home improvement store. The high-density foam will help reduce bubbles. A larger roller should be used for coating a larger canvas.
- Pour the coating into the tray and dip the roller in until it is thoroughly wet, but not too wet.
- Lay the graphic down on a clean, dust-free board that is bigger than the print. The extra space on the surface around the print can be used to roll off excess liquid if the roller becomes oversaturated.
- Apply two or three thinner coats, instead of one thicker coat.
- Don’t try to coat the whole print at once. Start with one or two passes at the edge. Go over each pass enough times for the bubbles to dissipate, but not so many times that the coating becomes tacky or bumpy.
- If large bubbles appear, try blowing onto the coating.
- Make overlapping passes to maintain a wet edge and avoid lines and streaks.
- Find a rolling pattern that works well. Some users prefer to go up the print in the first pass and down in the second, like mowing a lawn.
- Don’t press down on the roller. Maintain light, even pressure.
- Allow the first coat to dry before applying the second. A thin coat should take between 10 and 30 minutes to dry, but longer in a high-humidity environment.
- Allow the final coat to dry before packing the print for shipping. Don’t try to speed up the drying process with a fan; just allow the print to dry on its own.
- Clean the rollers immediately after each use, running cool water over them and then squeezing them until they run clear. Allow 10 to 15 minutes. Rollers that are kept clean can be used for about six months before needing to replace them.
- Wait at least six hours after applying the coating before stretching the canvas.
Generally speaking, coatings formulated for wide-format printing will work well with both aqueous and solvent prints. As more types of inks and canvas-based substrates are introduced, however, it is virtually impossible to predict the compatibility of every combination of media, ink and clearcoat. This is why testing is strongly recommended before actual use.
A test will immediately show if there are any incompatibilities, including water sensitivity, inkjet-receptive layer ‘mud cracks’ and ink bleeds. The tested material should be allowed to dry for at least 24 hours before evaluating the adhesion, flexibility and visual appearance. It should also be tested under simulated conditions similar to those to which the material will be exposed in real-world applications, so any expectations will be realistic.