The flattest form of masonite is called Duolux. It popular for half-assed photo mounting. Archival it ain't. If you simply use it as a backing material you can overlay it with Saran Wrap or some other barrier plastic. But that doesn't make much sense either, since fomeboard is cheaper and lighter. High quality synthetic boards which are pH neutral also need to be carefully chosen. Gatorbaord must be sanded. Ultraboard is extremely smooth. But applying a suitable acrylic adhesive foil to this kinds of substrates, then the print to that, is a tricky unforgiving application requiring special skills and equipment - best left to a pro shop. Suitable water-based glues are available from Seal and Daige, but likewise need both a high skill level and a suitable substrate/fiber-based print combination which will mutually absorb the water in the glue. Doing any of this requires homework and practice. Save up some scrap prints to test.