Good observation, Rick, but what you gotta keep in mind is that nowadays a lot of substitute materials are coming in from overseas and it's hard to know exactly what's in them sometimes. Even my dedicated plywood supplier couldn't give me a precise answer yesterday on an analogous question (some of these questions are code dictated locally). The mere thought of masonite etc would probably stop the pacemaker
of any real art conservator's heart. It might be fine for casual craftsy use, or temporary commercial displays, but certainly doesn't belong around anything of serious quality or intended permanence. It ain't all that flat either, which will be apparent if you try to mount a glossy
print to it. Ordinary fomecore isn't very smooth either. Sintra is so-so. Dibond... well, ya get what ya pay for, and you'll pay for that!
If you aren't opposed to a C-print, all of the major labs can print and mount it on 3mm styrene, which is very smooth, fairly rigid and totally inert. All of my studio work is printed and mounted this way.
I'm a retired cabinet maker, and I used to cut butt loads od modern MDF and melamine panels. I found I'm allergic to many of those products due to the formaldehyde, but never reacted to masonite. I would never mount a photo directly to the stuff personally, but possibly use it for a backing material behind something else. If someone where to want to mount a photo to it, I would seal it with a good blond shellac first then use a photo grade adhesive.
Yes, always true shellac, which is distinct from varnish. But that means you can't use a waterbased glue for the print. Another potential issue is the fact that sheet good mfgs for building materials aren't exactly monitored like a pharmaceutical concern - there can be a fair amt of residue cross-contamination from one product to another, including sulfur compounds. In our shop here we do cut entire pallet loads of masonite Duolux for a particular photographic organization (I won't say which one, because I think the stuff is a bad idea to begin with - but they only use it for temporary annual exhibitions). And I sell equip and supplies to everyone from high-end cabinets makers to these recent America's Cup teams with their carbon fiber concerns. But what you get allergic to just depends on the amt of exposure. The biggest culprit in this area is Port Orford cedar. The friends and clients of mine who were building Ellison's giant mansion out it just couldn't seem to get rid
of their "colds" month after month. Eventually skin allergies started popping up. It's a beautiful wood if you like that blonde Scandinavian
look or a true weathered silver gray, uncoated ... but hey, folks, some gloves and dust masks go a long ways to preventing this kind of thing.
Spray contact adhesives are a different story, however ... and frankly, you won't even have to worry about either the archival effects or
getting allergic, because once you get used to those glues, you'll either self-immolate from a spark, asphyxiate, or go brain-dead first like
every other gluesniffer!
There are a few different kinds of pressed wood board. Egg tempera painters (many of whom do think about how archival their supports are, due to the medium itself having a proven archival track record) are commonly using untempered masonite, which is as indicated above, just pressed pulp without formaldehyde or oil type additives. There is a layer of Gesso between the board and the paint, but it is the good old fashioned hide glue kind. So.... pressed wood > glue/gelatin > image, sounds familiar!
Unfortunately, untempered masonite is hard to track down, as suppliers aren't always aware of their products contents. If I can track down a current brand name, I will update.
I have personally never heard of a photographer mounting a print, an expensive one at that, on masonite — one of the least used and archivally appropriate methods of permanent mountingn for display or storage. Use Gatorboard (wood or styrene composite) or have the print hot-bonded to aluminium (di-bonding), then sealed.