The best mounting surface is foam core board. It comes in the same 32x40 inch sheets and is 3/8 inches thick. It is essential to get acid free foam core. If is even more important to have acid free foam core than an acid free mat since it is contact with the entire surface of the picture while the mat is only in contact with the edges. A possible alternative is to insert an acid free sheet of paper between the picture and the foam core. The very worst thing to use is the card board that comes with pre-made frames. This is the most destructive material possible for your art work and photos. It will yellow them in a year or less. See acid free mats.