More and more the painting was not looking too good, visibly falling apart even though it was restored in the not too distant past: we wondered if it was really clean because it should have been more colorful, and bubbles were forming where the canvas was detaching from the panel it was glued to. Not until we got it off the wall did we realize that it had been glued to plywood!
So, what's wrong with plywood you might ask? Well, both plywood and Masonite are very acidic materials and cause canvas and paper to deteriorate quickly if they are in contact. In addition to that super non archival product, the choice of a "permanent" glue (any glue you by at the hardware or construction supply store) and you can have yourself a real preservation mess!
As I write this, we are working very carefully to remove the canvas from the plywood in a way that will not stress the paint layers or kink the fabric. As you might guess, our main objective is NOT to get into a mess of flaking or raking paint. Here's a picture of a roll we use to help us handle the artwork as we finesse the canvas off the plywood.