Medium Density Overlaid Plywood can usually be special ordered from
any retail lumber yard. Georgia Pacific now makes most of it I think.
I used to buy a brand called Roseburg and there were two grades,
regular, and "2-step" which is smoother with less voids.
The coating which is described as "resin impregneated fiber" looks like
brownish paper.
I've built boats out of it to save a buck and it works BUT....the
quality of MDO seems to vary. Occasionally there are rather large voids
in it. Generally the 3/8 inch thickness has no or very few voids and the
1/2 inch has more voids. The 3/4 inch is worthless. So if you want, say,
a transom that is 3/4 inch thick,sandwich two thinner pieces together
with epoxy. Also, MDO is available with the "paper coating" on either
one or both sides...specified P1S or P2S.
This plywood is generally high quality douglas fir, just like marine but
with an occasional void.
There is also an HDO plywood that is used for concrete form work. I have
never used it but its worth a look if you have a source.
The trouble with marine fir plywood is that it checks badly unless epoxy
glass coated.
In any case, I'd cover at least the outside with epoxy and glass when
using MDO.

sel1 wrote:
Maybe someone out there knows what the trade name of this plywood is, but I
was recently talking to a guy restoring a 1953 sail boat and on the deck he
was using the plywood that they use for the large billboard signs. It is
made to withstand weather after all. Cheaper than marine plywood too. Says
its not the first time he used it (not on the same boat either).


"Mike" wrote in message
...

Can you use any kind of plywood for boatbuilding ort does it have to be
marine ply? Surely if you are planning on glassing and painting a plywood
hull it wouldn't really matter?