Premier Art Shield and Hahn's protective spray (I believe are the same) are the easiest to apply for light protection of photographic prints and matte fine art papers. The primary benefit of these sprays is their increased protection of prints from UV damage. Breathing Color's Timeless also contains UV inhibitors; Glamour II much less so. Btw, Jon Cone's gloss optimizer for piezography also provides some UV protection though that is not its purpose, and it does protect prints more along the levels of varnishes like Timeless or Glamour II.
As much as I dislike rolling (hvlp spray gun being much preferred), I have found that rolling Timeless is the best way to apply to fine art matte papers. It's a heavier protectant, adds a sort of softness in feel, but is a lot more work. Glamour II didn't work for me in this regard by either rolling or hvlp. 98% of the time, I'd go with the Premier Art Shield on fine art matte papers, especially if the final print is going to go behind glass. There is more artistic application and use with rolling Timeless on fine art matte papers.
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PremierArt Print Shield and Hahnemuhle seem to roughly double the lightfastness of prints as well as provide an impermeable barrier to increase resistance to gas fading and other oxidative agents. Given that the layer they impart is micron-thin, they also shouldn't impart any colour cast on the paper even if the polymer itself happens to turn yellow in a hundred years' time, and, due to its thinness, should also never delaminate. They're certainly not for protection against physical damage, though, and can't be used to change the surface texture of the paper.
Given their greater thickness, I would assume that Timeless, Glamour II and other similar varnishes offer much better physical protection than the spray cans, hence their use on unframed canvas. Timeless is stated to be 'non-yellowing' - has this actually been demonstrated through accelerated ageing studies? If a layer of Timeless turns yellow in a hundred years' time, it is thick enough to impart a colour cast on the print behind it. Also, given its thickness and lower flexibility compared to a micron-thin layer of Print Shield (yes, it's flexible, but you don't get much more flexible than a micron-thick polymer coating), would it be more prone to future delamination? On the other hand, it's stated to 'cross-link' with inkjet receptive layers (presumably the polyvinyl alcohol binder, which is also composed of long hydrocarbon chains) and allows canvases to be stretched and bent (where the inkjet layer of uncoated canvases would just crack when subject to the same treatment) so I'm not sure about this one.
Given that Timeless does not contain any levelling agents, how do you roll it smoothly? With a glop of Timeless on the printed surface, followed by smoothing it out with a glass rod? (a bit like coating paper with gelatin)
I certainly like the idea of using Timeless to achieve a gloss or semi-gloss surface on matte papers, though - particularly on uncoated papers, which are matte by definition - so, if things like yellowing or delamination aren't issues, it might be worth considering.