I have a big board and I it's MDF. I've had it for at least 5 years now. It just has a layer of batting type stuff and the canvas cover. Steam doesn't do anything to it. Even my super steam iron doesn't affect it at all.
A national quilt teacher said not to use that silver stuff. It is too slippery and can let your blocks get distorted more. Harriet Hargrave also says not to use that silver stuff in her books.