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!