I just ran across the following article in Robert Genn's older weekly letters. I have been painting on gessoed, tempered masonite, and noticed that others are painting on masonite as well (not sure if it's tempered or not), so I thought you may be as interested as I was in this article.
Untempered Masonite is indeed fading from
shelves. Further, the quality varies--corners can crumble away
like oatmeal cookies. Sealing and reinforcing with something
like acrylic medium doesn't always do the job. Some painters,
driven to tempered Masonite, have seen other alarms go off.
This support, while harder, is simply loaded with acids that
gradually seep through several layers of gesso. After a few
years the painting surface will certainly be foxed. The estate
of a deceased painter friend was mostly ruined by tempered
Glad I didn't sell most of those paintings. May have a couple of redos in my future!