So, does one need to spray or otherwise clear-coat pigment prints made on gloss paper? Is the situation different for B&W vs colour, or between Epson and Canon?
I think the answer depends a great deal on how concerned you are with both the aesthetics and longevity of the finished print. I dislike gloss differential and bronzing, and I want maximum longevity such that I routinely spray coat my finished glossy/luster prints. However, I know many well respected printmakers who are not concerned about those issues any more. They feel the latest pigmented ink sets impart enough longevity on all media which is sadly not true. The print permanence battleground has shifted more to the media properties as the inks have improved. They also think gloss differential and bronzing are not easily noticed any more, especially when the print is displayed behind glass or acrylic. Hence, they feel both issues are well under control. I disagree, and I would cite Canon's decision to add a clear coat channel to it's new WF printers as evidence that there's still plenty of room for improvement.
The use of coatings, whether sprayed, brushed, or laminated, have traditionally served two distinct purposes. They can be decorative or protective, and ideally (but not always) they impart both properties simultaneously. With respect to decorative properties, applying a top coating on some media can improve gloss, increase color saturation and tonal depth in the shadows. On other media, an additional coating may be applied in such a way that it is all but invisible, ie. neither improving nor degrading the initial image colors and tones, surface texture, etc, yet still imparting increased light fade, abrasion, and/or gas fade resistance. Yet with other inks and media there are serious compatibility issues (e.g., spray coatings and dye-based inks are essentially incompatible for initial image quality reasons even though lightfastness and gas fade resistance is increased). Much of the outcome depends on the interaction between the coating, the ink, and the media. Hence, much empirical testing is often involved to come up with coatings, media, and application methods that play nicely together and are convenient to use.
The printer manufacturers' attempts to improve gloss differential and bronzing via use of a clearcoat ink is very promising and delivers real improvements, but is still a bit of a work in progress, IMHO. It's important to note that the GO and CO technologies implemented by HP, Epson, and Canon, are all tuned for optimal coverage with RC media. They do very well with RC media, less well when applied on third party non RC glossy media, the so-called "baryta" papers. Those media in particular really suck up that clear ink so a thicker application than the printer typically lays down in one pass is warranted. However, the print maker typically has little or no control over the clear ink application other than what part of the print it will be applied, and feeding a print into the printer for a second pass is not practical for wide format machines although it's easy to do on a desktop printer.
Epson's GLOP, IMHO, is best in class, and the volume applied by the little SureColor P400 printer comes very close to eliminating all traces of gloss differential and bronzing on many glossy media, plus the coating is thick enough to be pretty effective on those third party Baryta papers as well. Canon's Chroma optimizer (CO), on the other hand, is aptly named because it is not nearly as effective in eliminating the differential gloss, but does greatly reduce bronzing and thus impart some "chroma optimization". If you set the Epson P400's driver settings to ensure full GO coverage all the way into the media margins, you can rub your thumb over the finished print margin area on a glossy microporous media and really feel a more slippery, fully sealed surface. It's great. With the Canon Pro-1, some change in surface friction can be felt, but the microporous ink receptor layer does not seem nearly as well sealed, IMHO. It is not as thick or glossy a clear coat compared to Epson's GO ink, and as such it is not quite as effective as Epson's GLOP
Based on my personal experience with the Canon Pro-1, I therefore don't know how useful the CO channel is going to be for me in the new Canon wide format printers unless these new WF machines give greater control on the amount of CO that can be applied. I suspect I will still have to resort to additional spray coating applications in order to finish my non RC glossy/luster prints the way I like them.
cheers,
Mark
http://www.aardenburg-imaging.com