I've used Moab Desert Varnish on some Baryta type papers (like Ilford GFS) have been pleased with the results. I find it helps with:
A) Gloss differential: This is the area of greatest improvement for me, and the primary reason I use it.
B) Overall Look: I feel like the print looks more "polished" overall when handheld, entirely subjective...
C) Physical Protection: The coating on Baryta and Platine papers can be delicate (varying, depends on brand/type) and the spray makes the print a
little more robust, again useful for hand holding/ passing prints around
D) I don't have any metric for if/how it affect print longevity
E) It doesn't appear to hurt image sharpness in any way.
Uses/issues:
1) I have only sprayed small (8.5x11 or 5x7) prints, which are manageable and not too pricey. Anything larger is framed behind glass.
2) Sprayed Baryta paper is glossy enough that displaying it without glass doesn't really reduce glare problems (IMHO).
3) I've never sprayed matte prints (I print very little matte) so I can't comment there.
4) The spray is a bit expensive, which is why I only use it for small prints, hand-held or displayed without glass. I have some small "frames" that are really just a fancy base with a 8x10 mat (5x7 cutout) on top that don't have glass and sit on a shelf.
5) Fibre papers usually have enough texture that I don't notice any "stippling" of the spray that I couldn't just chalk up to paper texture itself. I haven't had any problems with even-ness of coverage or running etc. It's actually really easy to do...
6) Make sure the print is dust free before spraying, spots of dust stuck in the coating are frustrating.
MS