Aw, c'mon guys. It's not dirty.
Face mounting is done in a cold-roller press. It is something like pressure-sensitive adhesive mounting, except that insead of applying a solid mounting board to the back, a sheet of clear acrylic is applied to the front.
Literally, the acrylic is glued to the face of the image. As you might guess, the pressure sensitive adhesive has to be transparent. And if anything ever happens to break, severely scratch, or otherwise permanently damage the face of the acrylic, you just throw it away and get a new one.
Andrew, I suggest you look for a company with a big roller press. Graphics companies or drafting supply/repro companies that create or reproduce computer graphics often mount them, too. Especially for large ones, that usually involves a roller press. If you can't find one that does it nearby, they'll at least know where to send you.