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From: "Craig Wall" <>
Newsgroups: rec.aviation.homebuilt
Subject: Re: Fly Baby - Marine plywood
Date: 02 Jul 1999 18:21:57 PDT

Ron Natalie <> wrote in message
> Craig Wall wrote:
> >
> > Ron Natalie <> wrote in message
> >
> > > Neither Marine or Exterior grade plywood is treated to be rot
> > > resistant.  The distinction from "normal" plywood is that the
> > > glue is waterproof.  You can get CCA PT plywood, but it's a
> > > different beast.
> >
> >   Exterior fir plywood and marine fir plywood use exactly the same
> > resorcinol glue.
> >
> Yes...that's my point.  What makes exterior grade exterior grade
> is the use of the waterproof glue, not because it is rot resistant.
> To be rot resistant it has to either use rot resistant woods
> (cypress, certain cedars, redwood) or use some kind of chemical
> treatment.  Neither of these is present in marine or exterior grade
> plywood.

  Ah. It sounded like you meant that exterior plywood was "normal plywood".

   Marine fir ply is the same as exterior grade except for the absence of
voids in the inner plys. And you are correct- neither is treated to be rot

   Craig Wall

From: "Craig Wall" <>
Newsgroups: rec.aviation.homebuilt
Subject: Re: Fly Baby - Marine plywood
Date: 03 Jul 1999 07:09:26 PDT

Bruce A. Frank <BAFRANK@**SpamBlock**> wrote in message
> I am not questioning the fact that the marine and exterior use the same
> glue but I have noticed something about the way they weather. When I was
> about twelve my father had a stack of reclaimed house wood (beams, 2X4s,
> and plywood) that he intended to use to build a backyard shop. The wood
> had been stacked haphazardly for years and my father one day began to
> sort and re-stack it. Some of it had rotted. Most all the exterior
> plywood was partially delaminated (this was South Carolina-plenty of
> year round rain--the plywood was stenciled "exterior BC"). I noticed one
> piece of plywood was in much better shape. My father said that it was a
> piece of marine plywood he'd had for more than ten years. The marine
> plywood in the same stack as the exterior grade was still sound and
> usable whereas almost none of the exterior grade was usable. The
> exterior plywood had been salvaged from waste from the construction of
> other houses in our neighborhood and was all younger than ten years.
> This was also back in the day when even exterior grade plywood had very
> few voids--in fact, I was fascinated by the fact that I could pull the
> layers apart and dis-assembled several sheets. All of the internal knot
> holes were patched in this exterior grade. (BTW, my father was not as
> "fascinated" at the internals of the plywood because my delamination
> party had spread slivers all over the lawn in the backyard. Even after
> my cleanup the lawnmower spit splinters for the next three months.)
> Was the glue used then(1960) different that that used today?

  No, it's the same resorcinol, but BC exterior is a cheaper grade than the
AA that you'd normally compare Marine grade Fir plywood to.   I suspect the
difference is (besides the obvious blems in the outer plys) that they don't
spend as much time compressing the plys while the glue cures.   Cheap
plywood is clamped in stacks and banded to provide gluing pressure, which
resorcinol needs to give a full-strength bond. (Also, the faying surfaces
might not have been sufficiently decontaminated to get good penetration.)
AA and Marine ply are usually clamped in stacks between plattens with
hydralic presses.

  Another reason might be that the laminations could be sufficiently
different in moisture content at the time of gluing in cheaper grades that
the differential swelling and shrinking could shear the plys at the
glueline.  Ideally, the plys would be in the same state when glued together,
so that the expansion and contraction would be synchronized to minimize

  I have built upwards of 30 small boats using AC Exterior Fir plywood and
have never had a problem with delamination, although dryrot is always
lurking if you fail to get all the water out during storage.  The DFPA
(Douglas Fir Plywood Association) does recommend doing a test on each batch
of plywood used for critical applications where water is involved, however-
it involves repeatedly boiling a test piece in water to check for

  Craig Wall

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