This is very bad idea since if the plexi gets scratched you will not have an investment or a happy collector. Plus if you are using lightjet or other non inkjet print you will have trapped the outgassing of chemicals in between the plexi and surface which will inherently speed up the fading of the print. There a no win proposition here. I know that Lik is doing this and will be sued for what he is doing. If you sign it and number it, the doors are wide open for the liability of the art. He will get sued for this. AIPAD would not do this or would any conservation dept of any museum do this. But hey if its for yourself go for it. Use a liner or matt, frame it the way it is suppose too. T
A properly processed traditional silver based print has no residual chemicals to speak of, and if done correctly has the correct ph to be neutral, and certainly has no outgassing. Most facemounting doesn't work very well with inkjet, using silicone processes will cause bleeding of the inks, and the surface isn't smooth enough for products like Seals optimount.
There seems to be a wide opinion as to what is acceptable and what is "archival". Each photographer needs to decide there own standards, but meeting museum standards is rarely necessary for ones work, and normally limits greatly the display options you may choose to use. Face mounting is certainly not new at all, and longevity of these images is far better than what many imply. In fact, some face mounting might improve print longevity ... Fatali comes to mind as his process somehow face mounts the traditional photo based print to museum glass, which basically seals out all outside influences from gases as well as protects from UV. It's actually a very cool look when framed ... the image seems to just float in space.
Lik does not facemount most of his work. I was in his LaJolla gallery a couple of days ago, and there were only a few images using plexi that may have been presented that way just for display in the store ... they were suspended back to back and not on a wall. I'm not sure he even sells prints this way.I've been told he does not use Plexiglass but uses thin Lexan (I suppose that may be a form of plexi, but certainly not what most photographers use when framing images). Most of the time the Lexan is between the liner and the frame, meaning it is not near the surface of the print. yes, this can scratch as well, but glass that size is a nightmare to work with and very few use it. I doubt very much if he will be "sued" ... I have a friend with a Lik print on the wall and it's still terrific - I'm not sure how old it is but it's at least 10 to 15 years. No clue if it is exactly the same as the day he hung it up, but it certainly doesn't appear to have any problem. I personally have images printed from negatives that have been displayed for over 30 years, and while I think there is some slight degradation, the image's still look just fine.