Another alternative
I've used MDO on one of my builds.
It is plywood with a very thin layer of mdf on the surfaces.
The glues used are graded for outdoors.
It is similar to plywood used for concrete forms, but has more,thinner layers and none I've seen has voids in the plys.
I've seen it used on ceilings of porches and it is also used by the Highway administration for signs because it is durable and the mdo surfaces make it great for paint and reflective stickers.
However it is harder to find.
I got mine thru someone I know who uses it often at his work to make forms for aluminum castings. They use it because of the smooth surfaces and can be abused.
Cuts much nicer than MDF.
www.insidewoodworking.com/mdo-and-hdo-plywood.html
I'm waiting for our contry to get on the ball and make Ecoboard more available.
It is available in the UK.
It is recycled plywood fibers that are molded with recycled plastics instead of glues.
It is supposed to be about 2/3 the cost of normal plywood, more moisture resistant than particle board, doesn't have the ill affects of milling/cutting like mdo.
By what I've read, it seems, lighter, denser, cheaper and more water resistant.
The Uk is promoting this due to the amount of wasted plywood in construction.
Especially for temperary walls you see outside of sites, then is thrown away because plywood's crappy resistance to water if not covered.