Some Plywood Info

[GregE]

Plywood is a topic that is asked about alot here. I emailed Greenwood Products about their XL panels, available through Overtons, and thought I'd post here in case anyone has the same question. I was curious about the pressure treated difference in CCA and ACQ. ACQ being the new method, and CCA being the former method no longer used across the board but only within marine and some other industries that are exempt from the laws.

My email to them...
"...Specifically, it appears that the CCA preservative is not the corrosive that most pressure treated plywood now is, but instead CCA is what all pressure treated plywood used to be. I assume that this is safe resting against aluminum, and safe with stainless steel fasteners, contrary to the ACQ pressure treatment method more commonly used now. Is this correct?...."

Their reply...
"We suggest that all users of XL? treated plywood, (both ACQ-treated and CCA-treated); use a barrier that will not allow any kind of contact to occur with Aluminum. Generally our manufacturers, the boat builders use a roofing tape, duct tape, or any other barrier that will prevent moisture moving through the tape itself while it is in service."

I guess always better safe than sorry. Interesting that manufacturers are using these simple methods to create the barrier. I would have thought the barrier would need to be a bit more substantial to hold up over time.

 

[45Auto]

Re: Some Plywood Info

Depends on how long you're talking about when you say "over time". It just has to hold up until the warranty expires ....

 

[GregE]

Re: Some Plywood Info

Depends on how long you're talking about when you say "over time". It just has to hold up until the warranty expires ....

Ahhhhh, good point!!!

 

[reelfishin]

Re: Some Plywood Info

The problem I've always had with any pressure treated plywood is it's weight, have you ever had carry a sheet or two of that stuff any distance?
I would be very cautious using either in contact with aluminum, CCA stands for "Chromated COPPER Arsenate", I would be afraid of a possible copper reaction with the aluminum.
I have always used plain, good quality outdoor grade plywood for all boat applications and I simply soak it with resin and coat well with glass.

I had a buddy that redid his 16' boat with all 3/4" CCA, the boat was already heavy for it's size, and he went ahead and did the entire floor, all seat bases, and the transom with the stuff. The result was a boat that no longer could use the brand new 50HP motor he had, top speed was reduced by about 12 miles per hour or more and the boat sat way too low in the water. He sold it somewhere, but I still don't think he knew what he did wrong. I am also sure that the foam was mostly soaked and he did nothing to remove that, he didn't even pull the old floor out, he just covered it up. I think he used about 6 sheets of that stuff and about 5 gallons of epoxy paint on everything.

Here's a site with some applications of the various type of pressure treating:
http://www.woodtreaters.com/WoodProducts/PressureTreating/ChangesToCCA.asp

 

[ondarvr]

Re: Some Plywood Info

The sheet of PT plywood is the same weight as normal plywood, it's the water in the wood that makes it heavy when you buy it. It needs to be dried first if you're going to apply polyester, epoxy or paint to it. The Greenwood XL sheets are already dry when you buy them.

 

[oops!]

Re: Some Plywood Info

rvr...

do you think it would help to scuff pt wood before applying a resin type product?

 

[gcboat]

Re: Some Plywood Info

Not to take any thunder away from ondarvr but yes. Some 80# paper and any decent orbital sander should do the job.

 

[ondarvr]

Re: Some Plywood Info

like GCboat said, a light sanding will help.

 

[oops!]

Re: Some Plywood Info

thank you all !

 

[tylerr83]

Re: Some Plywood Info

i was in the same position a couple weeks ago, trying to decide on what i was going to use for the deck in my 16' aluminum...after asking around here and talking to other people, i came to the conclusion that 1/2 or 5/8 ply with no voids in it (like a cabinet grade) sealed both sides/edges w/ epoxy resin should do the trick...i was gonna use 3/4 but either JB or TD recommended using something lighter so as not to weigh the boat down too much...like they told me...as long as a backing plate is used for seat bases it should be fine...

 

[fixb52s]

Re: Some Plywood Info

I bought some of that cabinet grade plywood for my boat. It was right next to the "marine grade" and seemed much better. The marine grade was $70 per sheet and the cabinet was $29. It is already smooth (I am going to sand it anyways with 80G), seemed about the same weight, and had more plys in it. It also seemed much more stronger.​

 

[tylerr83]

Re: Some Plywood Info

the big difference is that marine ply uses a special waterproof glue...but if its encapsulated w/ epoxy you're all set...the important thing is that the sheets don't have any voids in them where water can get in

 

[Coors]

Re: Some Plywood Info

Marine ply uses same glue as cdx

 

[steelespike]

Re: Some Plywood Info

No need to incapsulate pt ply.My pt transom ,77 Crestliner 18' utility,is 25 years old.In the water 8 months a year exposed year round for the first 20 years.No paint no sealer.
No doubt some paint would be good just to try to keep moisture even.
But I don't see any problems to this point.Factory transom lasted 5 years,
was totally shot.

 

[tylerr83]

Re: Some Plywood Info

true, the pt is already treated to prevent rot, i coudlnt' use that in my project because its aluminum, and i've heard that the chemicals used in pt can react w/ the aluminum and cause it to corrode in salt water

 

[reelfishin]

Re: Some Plywood Info

The sheet of PT plywood is the same weight as normal plywood, it's the water in the wood that makes it heavy when you buy it. It needs to be dried first if you're going to apply polyester, epoxy or paint to it. The Greenwood XL sheets are already dry when you buy them.

I have four or five sheets of plywood leaning against the wall in my shed, two of them are 3/4" pressure treated which were left over from a ramp I built about 8 years ago, they are still nearly double the weight of the same size sheet of cdx ply? How long does it take to dry out? They have been stored inside and are clamped to one wall to keep them straight. I do remember back when I built the ramp for the shed that they had two types of PT plywood back then, one was advertised as being a standard grade, and the other was recommended for saltwater use and had double the warranty or life expectancy. The saltwater use ply was far heavier than the cheaper one and was only a few dollars more. The better grade was 'wetter' and almost slimy feeling compared to the cheaper pt. The sheets I have indoors is still pretty much the same as when I bought them, they are still heavy and wet to the touch, while those that are outside have lightened in color some and don't see so wet, but they've been exposed to 8 years of weather and rain.
I did have to add traction strips to the ramp I built because every time it rained the pt would get slimy or slippery. I tacked down wood strips every few inches. Those are gone now and the wood don't seem to get that way when wet now. It has held up just fine though considering it sits right on the ground.
I seem to recall that they also sold another grade of treated ply as well that smelled like creosote, it was brown in color and sticky to the touch, it was recommended for "Environmentally sensitive applications and water contact". That was sold only at a Lowe's store near one lake I lived near back then. I stayed away from that due to the odor and lack of warranty.
The sheets I have out back are labeled 'CCA treated, 30 year warranty, Saltwater and ground contact rated on a small stapled on tag on the end of each sheet.

How long does it take for PT to dry out?
Maybe just the saltwater contact grade stays this wet?

 

[Robj]

Re: Some Plywood Info

Different species of wood, have slightly different weights. Pine is lighter, douglas fir is heavier. Not sure of the glue used on the cabinet makers ply, but exterior grade uses waterproof glue, something that you should be using in your boat. My motor mounts are aluminum, bolted with SS bolts to PT stringers, but there is several layers of glass in between. As far as weathering goes, I would take douglas fir ply over pine any day. Pine grows in a dry climate while doug fir in wetter climates, which makes it more resistant to rott. I would also try and stay away from ACQ ply for my boat and try to find the CCA treated.

Have a great day,

Rob.