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Old 23-08-2008, 06:57   #1
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Marine ply VS Pressure treated ply

Has anyone ever used pressure treated ply for building/repair on their boat? It would seem to me that pressure treated would last longer as it is made for wet/ground contact and it won't rot. Does epoxy stick to it?
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Old 23-08-2008, 07:01   #2
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More importantly is the glue they use in marine ply. The cost of the panel is minimal compared to the effort you will make to install it. Buy the best you can.
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Old 23-08-2008, 07:41   #3
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I recently toured Parker Boats factory here in NC. The wood they are using between fiberglass is pressure treated. Now it may be specially made for marine use with different glue, but Parker boats are nearly "bullet proof", that is they last a long time and take rough water well. A lot of law enforcement agencies and fish and game folks buy them.

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Old 23-08-2008, 07:58   #4
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More importantly is the glue they use in marine ply. The cost of the panel is minimal compared to the effort you will make to install it. Buy the best you can.
I do know that pressure treated wood has waterproof glue and is made to be used in wet locations. It is more expensive but I was wonder how epoxy/glass bonded to it. It also weighs a little more due to the treatment.
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Old 23-08-2008, 08:00   #5
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Most of the pressure treated plywood (as well as dimensional lumber) I have seen is made from southern yellow pine. Polyester resin does not like the resins in yellow pine and thus has some adhesion problems. I would recommend not using it with polyester based resins. I have not had any problems using epoxy with pressure treated anything. That was allowed to dry thoroughly before using.

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Old 23-08-2008, 08:14   #6
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I have not had any problems using epoxy with pressure treated anything. That was allowed to dry thoroughly before using.
This is important. A lot of pressure treated, including plywood is delivered to the stores and lumber yards wet.
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Old 23-08-2008, 11:24   #7
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One of the inherent advantages to Plywood is the excellent weight to strength ratio of DRY wood

If you start loading it up with liquid preservative (arsenic), you negate that advantage.

Bonding problems can also be a concern

Marine Grade Plywood is specified as such due to the superior quality of the ply veneers (fewer footballs, checks, voids etc.) The glues used in the lamination process are the same as those used in Exterior Grade Plywood

As for the pressure treatment and rot prevention, IMO you're better off striving for a strong, light, DRY wood core;

And trying to keep it that way with superior surface protectants (composites, sandwich construction, resin saturation, paint etc.)
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Old 23-08-2008, 13:04   #8
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The quality of the pressure treated ply is not as good as marine and has many voids which are to be avoided. Most of what I see is rated CDX. The first two letters refer to the quality of the face veneer and the third stands for exterior glue. I think real marine grade ply is rated by a different standard but the construction grade ABX would be similar. I do not like working with pressure treated lumber, cutting , handling, or sanding due to the chemicals used though they are safer than in the past.
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Old 23-08-2008, 13:45   #9
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I would not chance it on the cheaper stuff. Think about the consequences and the subsequent costs if it fails versus the additional cost of going with marine ply.

Do you know exactly what Parker uses and how the naval architect came up with the scantlings for their boat? We are not talking your boat here.

I think it is too high of a risk/reward ratio to go with the cheap stuff.
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Old 23-08-2008, 13:54   #10
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The quality of the pressure treated ply is not as good as marine and has many voids which are to be avoided. Most of what I see is rated CDX. The first two letters refer to the quality of the face veneer and the third stands for exterior glue. I think real marine grade ply is rated by a different standard but the construction grade ABX would be similar. I do not like working with pressure treated lumber, cutting , handling, or sanding due to the chemicals used though they are safer than in the past.
Yes, Steve

Hence Marine Ply is designated AA

Many plywood boat builders use ABX ply under the assumption that they are only sacrificing the quality of one face veneer (A for B) in favor of a significant price difference between AA Marine Ply and ABX.

But ABX Ply also has more voids and checks in the core veneers, as well as footballs in the face

The glues, however, are the same for both AA Marine and ABX

Every few years the Plywood Rating Assoc. lowers the acceptable standards for AA Marine PLY. (allowing more and larger defects in the plys)

Back in the '60s, it would have been considered sacreligious to have AA Marine Ply with ANY face or core defects or voids (footballs were out of the question)

Marine Ply is also usually stored inside (out of the elements) at lumberyards
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Old 23-08-2008, 13:55   #11
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Exterior plywood uses the same glue as marine plywood but the interior veneers often have voids. Marine plywood uses a much better grade of veneers with no voids and also may use more plys of laminate. The variety of the wood itself may be the same but the quality of the wood is way better in marine ply. Most production boats use a good quality exterior grade plywood for their interiors, typically ABX or AAX.

You can get exterior grade plywood with mahagony, teak or a lot of other types of wood for face veneers. The interior is going to be fir, however. Don't know whether Marine grade plywood has the exterior face plywood as the core plywood or whether it's just a higher grade of fir.

FWIW, I used ABX and AAX for the interior of our Westsail. It was quality plywood with very few voids, cost quite a bit less than marine grade but quite a bit more than they typical exterior grade plywood at the lumber company. It has held up well for more than 30 years. Of course the key to plywood longevity is keeping it reasonably dry. Any plywood will delaminate if constantly immersed.

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Old 23-08-2008, 14:07   #12
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Has anyone ever used pressure treated ply for building/repair on their boat? It would seem to me that pressure treated would last longer as it is made for wet/ground contact and it won't rot. Does epoxy stick to it?
If you understand the dynamics of the W.E.S.T. system;

It is hard to rationalize using plywood treated with anything (other than epoxy).

The whole premise is to keep the water (moisture) OUT of the core.

A wet core is substantially weaker (even if it's not rotten) and will eventually lead to potential delam problems
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Old 09-09-2008, 12:26   #13
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many of kids playground equipment has ben taken down because of the chemicals used. i wonder about off gassing inside the boat? just a thought.
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