Don't use regular mdf. It is not an outdoor material. They now make a mdf product that claims to be for outdoor use, but it doesn't have a track record, and I would not trust it until is has proven itself. Anyone remember the fantastic marketing claims of Masonite brand siding in the 70's? Literally tens of millions of feet were installed on houses before it all starting disintegrating from moisture a couple of years later.
Like suggested, locate some mdo. It is tough stuff and very smooth finish. Next choice would be a piece of MARINE grade plywood coated with three coats of epoxy resin.
