Do not use MDF! It will only hold up as long as the finish does and it has very little strength and wont hold fasteners very well. If you don't want to use Marine Ply, try using MDO (medium density overlay) It's exterior rated plywood with a smooth paper face, sign makers use it for all types of exterior applications and it's available in most lumber yards.
If you need it smooth on both sides you'll have to make sure you ask for it, otherwise it's smooth one side only.
CDX is also a bad choice as it's a framing lumber ment for sub floors and wall sheathing. It's prone to warping and has lots of voids in the layup. Ac is an interior grade Ply and they make it smooth by plugging voids with "foot balls" no matter how much you paint these will always show through the finish.
Last edited by Sabre66; 03-31-2007 at 10:04 AM.