Plywood Clarification Please - Marine/Exterior Grade

Plywood Clarification Please - Marine/Exterior Grade

Post by Bugs Bunn » Tue, 11 Dec 2001 20:19:32


I see this type of question from time to time and need some clarification.

Marine grade compared with Exterior grade.  I have read that marine grade
has no voids and some times you have voids with exterior grade.  The big
thing that I have read said that both marine grade and exterior grade now
use the same glue.  This would make the void issue the only difference
(along with the costs).  Is this true?

The other big concern I have is with epoxy.  Will epoxy properly bond with
Exterior grade?

I do not consider myself cheap, however if I plan on putting a layer of
fiberglass and epoxy over a plywood deck including 2 coats of epoxy on the
underside, is it really necessary to use marine grade.  The project is the
restoration of a 33' Silverton boat that I will use for fishing the gulf
stream.

Please note this is my first project, so please do not slam me too hard for
the exterior grade question.  Thanks in advance for all the opinions on this
plywood clarification question.

 
 
 

Plywood Clarification Please - Marine/Exterior Grade

Post by David Carnell » Tue, 11 Dec 2001 21:14:01

Marine and exterior have always used the same glue and passed the same boil
tests.  Marine has more plies for a given thickness.  It is supposed to have
higher quality veneers and fewer voids.
Quote:

> I see this type of question from time to time and need some clarification.

> Marine grade compared with Exterior grade.  I have read that marine grade
> has no voids and some times you have voids with exterior grade.  The big
> thing that I have read said that both marine grade and exterior grade now
> use the same glue.  This would make the void issue the only difference
> (along with the costs).  Is this true?

> The other big concern I have is with epoxy.  Will epoxy properly bond with
> Exterior grade?

> I do not consider myself cheap, however if I plan on putting a layer of
> fiberglass and epoxy over a plywood deck including 2 coats of epoxy on the
> underside, is it really necessary to use marine grade.  The project is the
> restoration of a 33' Silverton boat that I will use for fishing the gulf
> stream.

> Please note this is my first project, so please do not slam me too hard for
> the exterior grade question.  Thanks in advance for all the opinions on this
> plywood clarification question.


 
 
 

Plywood Clarification Please - Marine/Exterior Grade

Post by Stev » Wed, 12 Dec 2001 01:37:31

Another consideration. Exterior will generally have a very thin "A" or "B"
side veneer and if you using this for a deck, you run the risk of 'punching'
thru the veneer when walking on it or when fastening down deck hardware. I
would recommend putting the "C" up if you use exterior (the "C" side veneer
is always thicker and you can just fill the surface voids before glassing).
This also will give you a nice clean veneer on you interior overhead.

I use what is called SuperPly which is exterior plywood that has mahogany
veneer on the "A" side. The mahogany veneer won't 'print' through when you
paint the underside.

Be careful when purchasing exterior ply wood since they are really getting
some "Cheap Crap". Make sure your getting at least 5 ply. Don't except the 4
ply crap. The exterior plys are really thin and the interior plys are thick
and run the same direction. Always insist on a odd number of plys to avoid
parallel adjacent plys.

My experience and opinion. FWIW

Steve
s/v Good Intentions

 
 
 

Plywood Clarification Please - Marine/Exterior Grade

Post by William R. Wa » Wed, 12 Dec 2001 05:55:11

You can use construction grade plywood for the deck. If its made of fir
you need two layers of fibreglass on the top surface to keep the wood from
spliting in the hot sun. Epoxy resin sticks fine to construction, ie
exterior grade, plywood. I belive polyester sticks better to the softer
fir plywoods than it does to the harder tropical woods. However, most
people would pay extra and use epoxy.

The following provides some basic info on what kinds of plywood to use for
the exterior surfaces of boats. I'll put a copy on my web page.

........................................................................

Plywood for boat building comes in two basic grades, construction
(or exterior) and marine. It also comes in two basic  woods, wood
from temperate climates (fir) and wood from tropical climates
(often called mahogony). The woods used come in two basic ages,
old growth and second or later growth.

Marine grade is superior to construction grade. There is more
glue, more layers, and fewer imperfections.

Tropical climate wood is harder, denser, and stonger than
temperate climate wood because there is no difference in growing
rate throuhgout the year. In temperate climates the wood grow fast
in summer and slow in winter.

Old growth woods are denser and stronger becuase the trees grew
slowly in dense shade. Second and later growth trees grow faster
on cut over land. The annual growth rings in old growth trees are
closer together and the cells packed closer together.

There are different approaches for each kind of wood for long boat
life. One approach is to use tropical marine plywood, seal the
edges with plastic resin or with glued wood strips, and finish
with paint or varnish. Another approach is to use construction
grade plywood and seal all the surfaces with plastic resin. The
choice is a matter of cost and availability, marine plywood and
plastic resin both being rather expensive. When using either
plywood made from temperate climate wood (fir) all surfaces should
be covered with plastic resin and fibreglass cloth for longevity
because the wood exapands and contracts and develops surface
splits especially when exposed to the heat of direct sunlight.
However if longevity is not an issue and a inexpensive boat is
desired it can be made of construction grade plywood finished with
paint.

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Plywood Clarification Please - Marine/Exterior Grade

Post by Chris Cranda » Wed, 12 Dec 2001 06:44:47


: The following provides some basic info on what kinds of plywood to use for
: the exterior surfaces of boats. I'll put a copy on my web page.
: ........................................................................

: Plywood for boat building comes in two basic grades, construction
: (or exterior) and marine. It also comes in two basic  woods, wood
: from temperate climates (fir) and wood from tropical climates
: (often called mahogony).

Of course, pine plywood is also widely available in the USA, and can be
used from some boatbuilding purposes.  It's from a temperate climate
(generally, although north Florida in August doesn't seem temperate), and
it's heavier and often cheaper than fir.

 
 
 

Plywood Clarification Please - Marine/Exterior Grade

Post by Backyard Renega » Wed, 12 Dec 2001 19:00:50

Quote:

> the exterior grade question.  Thanks in advance for all the opinions on this
> plywood clarification question.

WHAT AN IDIOT!...just kidding...For what you describe I would use the
Marine plywood. Exterior is fine for some projects as the others have
said for small projects or decks (as long as you turn it into a
plastic composite first) but for an off shore boat, I would use
Marine. Yes the glue is the same, but that's where the similarities
end, you can pay now or you can pay later, Scotty
 
 
 

Plywood Clarification Please - Marine/Exterior Grade

Post by Credence Vision Systems LL » Sat, 29 Dec 2001 23:47:47

Marine ply has tighter specs for fewer voids, although lately the difference
between exterior and marine seems to be diminishing.  Marine ply also has
tighter specs for glue coverage.  It is more likely to be properly glued
than exterior, e.g. glue voids are weak areas are more prevalent in standard
exterior grade ply.

With that being said, epoxy coating everything prior to installation solves
99.9% of the issues.  You can build with exterior fir if you want.  A bigger
issue to me is workability ... working with fir plywood is harder.  It has
more sap and more voids.  If you are going to pay for marine plywood,
consider some of the low cost mahogany products that cost the same, e.g.
Aquatek (more plies than fir, no voids ... even though it is BS6588 and is
allowed some ... it has none, stiffer, greater impact resistance, etc.)
Aquatek is Okoume, a type of mahogany, and superior in every way to the
marine fir plywood.  It costs within a dollar of marine fir plywood.  Don't
know all the places you can buy it, but Edensaw has it
(http://www.edensaw.com) ... no affiliation whatsoever.  They ship.

Later,
Brian


Quote:
> Marine and exterior have always used the same glue and passed the same
boil
> tests.  Marine has more plies for a given thickness.  It is supposed to
have
> higher quality veneers and fewer voids.


> > I see this type of question from time to time and need some
clarification.

> > Marine grade compared with Exterior grade.  I have read that marine
grade
> > has no voids and some times you have voids with exterior grade.  The big
> > thing that I have read said that both marine grade and exterior grade
now
> > use the same glue.  This would make the void issue the only difference
> > (along with the costs).  Is this true?

> > The other big concern I have is with epoxy.  Will epoxy properly bond
with
> > Exterior grade?

> > I do not consider myself cheap, however if I plan on putting a layer of
> > fiberglass and epoxy over a plywood deck including 2 coats of epoxy on
the
> > underside, is it really necessary to use marine grade.  The project is
the
> > restoration of a 33' Silverton boat that I will use for fishing the gulf
> > stream.

> > Please note this is my first project, so please do not slam me too hard
for
> > the exterior grade question.  Thanks in advance for all the opinions on
this
> > plywood clarification question.