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Author Topic:   Marine plywood
NEVER SCARED posted 03-01-2002 03:58 PM ET (US)   Profile for NEVER SCARED  
Hows everyone in Whalerland? I was wondering what is so special about marine plywood? I always thought it was treated to resist rot, but rotted transoms are common. I also hear that the ONLY difference between mp and regular grade plywood is the absence of voids. Whats so special about that? What are you paying extra $$$$S for? Also does anyone know if you can get 3/4" pressure treated plywood for replacing the deck on our BWs?

Thanks in advance

OutrageMan posted 03-01-2002 04:23 PM ET (US)     Profile for OutrageMan  Send Email to OutrageMan     
You are right about marine grade plywood not having voids. What IS special about that, is that it does not give the water a place to accumulate and cuase rot. You are paying the extra money for that. Well worth it in my opinion.

I am not sure if you can get pressure treated ply. I am not sure why you would need to. You can do a much better job of making it rot resistant by "painting" it with epoxy before you install it.

Chesapeake posted 03-01-2002 04:44 PM ET (US)     Profile for Chesapeake  Send Email to Chesapeake     
Yep. You can get pressure treated green plywood. They have it at Home Depot. It is very heavy and not cheap... but still less than marine ply.

I had always been under the impression that marine ply also used a different glue between layers. Is this not true? If not, regular outdoor ply, with edge holes filled with epoxy and "epoxy painted" should be just as good, if not better... Yes? As I recall, even marine ply is not free from such gaps. I recall filling such with thickened epoxy.


JBCornwell posted 03-01-2002 05:09 PM ET (US)     Profile for JBCornwell  Send Email to JBCornwell     
Ahoy, Never.
I believe marine grade plywood is mostly void-free and glued with waterproof glue, but not bug-proof.

Pressure treated plywood is also glued with waterproof glue but is insect proofed and rot resistant. It is heavy. It does not finish well and the obverse is usually very ugly.

I don't know if it is still available, but my Dad used a lot of A-A marine cabinet grade mahogany plywood. Waterproof, rot resistant, 12 or 16 plys per inch, zero voids and furniture quality mahogany veneers on both surfaces. Stronger than ash and heavy as hickory. Cost a ton.

The point is that if you look in the right places you can get plywood with whatever qualities you desire. Cabinet makers can point you to the right places.

In an application that doesn't need to be very strong and doesn't need to take a finish exterior grade treated plywood is as good as marine plywood. . . maybe even better.

Red sky at night. . .
JB :)

NEVER SCARED posted 03-01-2002 06:35 PM ET (US)     Profile for NEVER SCARED    
Thanks guys! My deck is still solid, but theres 2 holes where cooler chocks were that I removed and the holes are wet. Im hoping I can dry them out and just fill with epoxy, but if I have to redo the wood under the deck, I want to use something thats gonna last!
Tom W Clark posted 03-01-2002 08:38 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
Marine grade plywood differs from exterior grade plywood only in the quality of the veneers. The glue used in both is waterproof. Marine grade plywood is supposed to be void free. In reality marine grade plywood sometimes does have voids. By the same token, exterior ACX or ABX sometimes looks as good or better than marine grade.

Not all units of plywood are the same even though they are supposed to meet certain criteria. It is possible to get good ACX at the lumber yard. Just keep your eyes peeled for nice looking veneers and void free edges. But even then, that's no guarantee. But again, let me reiterate, the glue is the same.

The fact is, plywood is a wood product and as such is subject to the quality of the trees the veneers are peeled from. A few decades ago the plywood that was commonly available was much nicer looking stuff. Nice old growth Douglas fir produced beautiful tight grained veneers, far fewer knots. Those days are gone. But the good news is that glue technology has come a long way. The glues used in today's plywood is MUCH better than in the old days. The stuff is truly waterproof.

Regarding pressure treated plywood, let me point out that the stuff you see at the lumber yard is pressure treated CDX, a much lower grade. I have never seen pressure treated ACX. Perhaps you can get it but I've never seen it.

Pressure treated plywood (and lumber) is treated with a preservative which is water borne and thus is applied to really wet lumber. This is why it is so heavy and wet when you buy it. If you are going to put it in a boat you best make sure you've dried it out first. Perhaps there is a boat grade of p.t. plywood that is dried. I've just never seen it.

If you're trying to avoid having plywood rot there are really two distinct ways to go about it.

1) Don't let the plywood get wet. No water, no rot.

2) Chemical preservatives., like the CCA in p.t. lumber and plywood. The idea is the stuff can be wet and still not rot but this is not forever. Pressure treated lumber does rot, I've seen plenty of it over the last 20 years in spite of their typical fifty year guarantee.

whalerron posted 03-01-2002 09:50 PM ET (US)     Profile for whalerron  Send Email to whalerron     
Be careful using pressure treated plywood in a boat if the wood cannot "breathe". This is because most pressure treated plywood has not been "redried" enough for use in a boat. Here are 2 very good links.

Marine Plywood:
Pressure Treated
Plywood in a boat:
General use of lumber in boats:

- ron

Duncan posted 03-03-2002 07:05 PM ET (US)     Profile for Duncan  Send Email to Duncan     
Pressure treated Plywood (CCA or Coper, Chroimum,Arsnic). Most is Southern Yellow Pine and if not secured flay will warp like a SOB. Grade usually allows large knots in the veneers as long as they are sound but they likely won't stay sound if you walk on it over time. Fine if you can patch and paint.

A note on HOME DESPOT...their wood is crap. They chisel on grade constantly. Their #2 pine is shop grade at best. If you're gonna put your labor into a project (particlularly a whaler) pay a real supplier a couple bucks more.

Lil Whaler Lover posted 03-04-2002 07:30 AM ET (US)     Profile for Lil Whaler Lover  Send Email to Lil Whaler Lover     
If you need plywood for a boat look for Greenwood Industries XL-50 plywood. This is pressure treated and for marine use. The deck of my Crest pontoon boat is made from it and carries a lifetime warranty against any failure of the plywood. The manufacturer has been in business 45 years (they have a clue what lifetime means!) and used this product for 15 years. During that time it has performed perfectly with no warranty claims. They also use it for their furniture bases with a lifetime warranty. Most of their competitors use roto-molded plastic seat bases and offer as little as a one year warranty on that stuff.

Not only does Crest offer the warranty but Greenwood also offers a lifetime warranty on its product.

Bayliner (not a good word on this site) switched to Greenwood XL-50 on its Capri models from Sitka spruce and increased its hull warranty from 5 years to a lifetime warranty, transferrable to the second owner within the first ten years of ownership.

This is a ood product if you can find a source for it.

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