I mostly use rubber, fibre-glass or plaster, but imagine that plywood would have much the same properties as plaster when casting concrete.

My guess is that the moisture in the concrete would cause unsealed plywood to swell, which would give you problems when you try to release it. I'd seal all surfaces of the plywood, and coat the inner surfaces with a release agent (like vaseline, or vegetable oil).

I have used plywood a few times. Make sure it is sealed very, very, very well. Runningdog is right the moisture will effect the wood. Plus a good release agent helps.
Paws

Michele,
My uncle uses plywood forms. He has done a couple different things;
1) paint with cheap latex paint, and unmold at the earliest time possible. Scrape down your mold do not spray with water
2) paint with cheap latex paint and use oil (motor oil) to paint over the mold. Unmold and clean your mold with as little water as possible
3) paint the mold with urethane, fiberglass resin, or shellac it really good. Again should use a release agent of oil or a manufactured release agent.
Cleaning your molds as soon after as possible and with as little water will keep them going for a long time
Happy casting,
Dena

To avoid wicking I just surface the wood with glued on heavy 6 mil plastic and it's good for life. Very smooth, no wrinkles, almost impossible to tear.
You can buy a roll of 6 mil vapor barrier plastic anywhere, it's used in construction of insulated buildings.
It's usually either folded 8 foot or 10 foot and a $15 roll will last you a very long time. This is the cheapest way to make water proof plywood forms that I know.

I should mention that I also use a stapler and just staple the plastic to the plywood.
Cheaper than glue and works as well.

I just wanted to say a big thank you to all who answered my questions. It gave me a lot of food for thought...and things to try.

You are all great!!

thanks
michele

I used plastic from empty peat moss or sand bags. It is plenty strong. I stapled it to the inside only of the form before putting in the screws. I use blocks of styrofoam, wrapped in plastic for the inner form.