Varnish coatings are available in gloss, satin or dull finishes, with or without tints. Varnishes offer a relatively low degree of protection compared to other coatings and laminates, but they are used widely, thanks to their low cost, flexibility and ease of application. Varnishes are applied just like an ink, using one of the units on the press. Varnish can either be flooded across the entire sheet or spot applied precisely where desired, to add extra gloss to photos, for example, or to protect black backgrounds. Although varnishes must be handled carefully to prevent the release of harmful volatile organic compounds into the atmosphere, when dry they are odorless and inert.
In addition to providing relatively little protection, varnishes have other drawbacks too. One problem is that over time, they tend to yellow. Yellowing is not a big concern when the varnish is used over process colors, but it is noticeable when the varnish is applied over unprinted paper, especially today’s high-brightness blue-white papers. And for some reason, silk, dull and matte coated papers tend to reveal yellowed varnish much more than their gloss-coated counterparts. In an attempt to counteract the yellowing, some printers will add a small amount of opaque optical whitener to the varnish that will be applied over white paper, but it is a less than perfect solution.
Varnishes also require the use of printers’ offset spray powder to keep the printed sheets from sticking together before the varnish is completely cured. The powder that is left behind can affect the look and feel of the finished piece, an especially important concern in fashion catalogs and other publications where appearance is everything.
Low cost water based aqueous coatings are among the most commonly used coatings available today and provide good protection from fingerprints and other blemishes. Like varnishes, aqueous coatings are applied in-line on press, but they are shinier and smoother than varnish, have higher abrasion and rub resistance, are less likely to yellow and are more environmentally friendly. Aqueous coatings dry faster than varnishes too, which means faster turnaround times on press.
Available in gloss or dull finishes, water based coatings offer other advantages as well. Because they seal the ink from exposture to the air they can help prevent metallic inks from tarnishing. Specially formulated aqueous coatings can be written on with a number two pencil, or overprinted using a laser jet printer, a key consideration in mass mail projects.
Since they are less likely to yellow, aqueous coatings provide a good alternative to varnish, especially when it comes to protecting projects that feature large amounts of white space. However, it is difficult to apply a spot aqueous coating with the same degree of precision that is possible with a varnish, which is why aqueous coatings typically are flooded across the entire sheet. Because the coatings are typically applied over the entire sheet and are water based, most experts recommend using 80# text weight or heavier paper stocks to keep the paper from becoming curled or wrinkled. Aqueous coatings can be used in conjunction with either varnish or UV coatings, but doing so can be costly, and unless production is managed carefully, the coatings may not dry or lay right.
Aqueous coatings and UV coatings are also susceptible to chemical burning. In a very small percentage of projects, for reasons not fully understood, certain reds, blues and yellows, such as Reflex Blue, Rhodamine Violet, Purple and PMS Warm Red, have been known to change color, bleed or burn out. Heat, exposure to light, and the passage of time can all contribute to the problem of these fugitive colors, which may change at any point from immediately after the job leaves the press to months or years later. Light tints of colors, made using a 25% screen or less, are especially prone to burning.
To help combat the problem, ink companies now offer more stable, substitute inks that are close in color to ones that tend to burn, and these inks are often used to print light tints or bright colors. Even so, burning can still occur and dramatically affect the look of the project.
Extremely high gloss UV, or ultraviolet, coatings offer more protection than either varnish or aqueous coatings. UV coatings are applied in-line by printers or, more frequently, off-line by printers, finishers or converters. UV coatings are applied as a liquid, using a roller, screen or blanket, and then exposed to ultraviolet light to polymerize and harden the coating, with zero emissions. The coatings can either be applied across the entire page or, while lacking the precision of a varnish, on a spot basis. The coatings are available in a high gloss as well as matte, satin and a wide variety of specialty finishes, including glitter and tints, and even different scents.
When first introduced, UV coatings were often perceived as murky and yellow. Those problems have been corrected, but the coatings still have other drawbacks. Like aqueous coatings, UV coatings are susceptible to chemical burning. UV coatings also are more likely to show fingerprints than either aqueous coatings or varnish, and some UV coatings can make paper difficult to fold. Dwell time, viscosity, temperature, the intensity of the UV light source and the interaction between the coating and the paper can all cause problems too. If conditions aren’t right, the coating will not have a chance to spread evenly across the page before the coating is cured, and the finish can develop an “orange peel” look.
UV coatings also tend to accentuate the roughness of, or any defects in, the surface of the paper. Some printers insist that UV coatings require the use of coated paper stocks because uncoated papers allow the coating to sink into the sheet, leaving little of it on the surface. And in the past, most printers said that UV coatings should be used in conjunction with UV inks and that if conventional inks were used, they had to be wax free and allowed to dry completely before the coating was applied. However, new hybrid inks help reduce the potential for drying and surface problems.