Plywood - pressure treated vs marine grade?

[VHarris001]

I've bought a lot of pressure treated plywood over the years and had varying
degrees of success. Some peices warp badly. Also the surfaces can be pretty
rough, and they get much worse with age unless painted.

Anyone have experience with marine grade plywood? Does it warp? Can I get it
in either AB or AA surfaces? Will the surfaces hold up in outdoor use without
painting/sealing? Any other benefits/problems with marine grade, other than it
costs more than double treated plywood?

TIA
Vernon

[dav...@myremarq.com]

PT plywood simply means that is has been treated for ground contact. (less
susceptable to damage from rot and termites)

Marine plywood is designed for strength. (more laminations, exterior glue,
lack of voids) Navy PT boats were made out of plywood.

In either case you should apply a finish to help preserve it's looks.

**** Posted from RemarQ - http://www.remarq.com - Discussions Start Here (tm) ****

[an...@mindspring.com]

VHarris001 wrote in message
<19990510185348...@ng-fh1.aol.com>...

If I remember correctly, marine grade means there are no voids in the plys
as opposed to the rot resistance of treated.

I would stick with the pressure treated.

To keep it from warping, you need to dry it in a controlled atmosphere. As
if everybody has a drying kiln in the back yard!

I guess the real question is, what are you building?

a.

[VHarris001]

I'm putting sides on a utility trailer that gets hard daily use and is stored
outside in the weather. Must have a smooth (A) and should have at least a B
interior. I put exterior AB plywood sideboards on a pickup truck last year and
it's also warping. The hardware store said the only thing I could do was dip
it in a sealer (Thompsons, etc.) but didn't know anyone locally who could do
that for me.

[VHarris001]

Does that mean that marine plywood has not been treated for ground contact? Is
it no more resistant to moisture than standard plywood?

[dav...@myremarq.com]

Based on that application I wouldn't waist the money on PT plywood. Marine
is good quality but you could save a lot by getting an AC with exterior
glue.

You need to seal the plywood with a paint or high quality sealer to prevent
the constant expansion contraction that comes from getting wet and drying.
That's what's causing your warpage and probably will continue to happen
with PT or Marine grade. (maybe a little less with Marine due to the
quality ply's)

(Don't waist money on Thompsons wood sealer, it's garbage. You weant
something that will prevent moisture penetration.)

[rugeeky2?]

I dunno, I used Marine grade for a utility trailer, and it worked great.
Part of it was used for the bottom, and I actually used this trailer to dunk
my 12' boat. The trailer was NEVER garaged, and we have some tough weather
here. No warping, no splinters, and the sucker is still holding up well
(although its with the ex-wife now, sniff-sniff).

The bottom line question is would I do it again. The answer is absolutely!

TinMan1332 wrote in message <19990510211005...@ng25.aol.com>...

>>Based on that application I wouldn't waist the money on PT plywood.
Marine
>>is good quality but you could save a lot by getting an AC with exterior
>>glue.
>>
>>You need to seal the plywood with a paint or high quality sealer to
prevent
>>the constant expansion contraction that comes from getting wet and drying.
>>That's what's causing your warpage and probably will continue to happen
>>with PT or Marine grade. (maybe a little less with Marine due to the
>>quality ply's)
>>
>>(Don't waist money on Thompsons wood sealer, it's garbage. You weant
>>something that will prevent moisture penetration.)
>

>Good advise. Add to that that PT plywood is all pine (and born to warp).
>
>J.P.

[dav...@myremarq.com]

The edges of plywood are very vulnerable. I would cover them with molding.
(either 1/2" X 3/4" molding or even plastic T edging)

I would either paint it with several coats of oil based enamel or spar
varnish. If you want to use a natural sealer than I'd use Sikkens teak oil,
Varathane #66 natural oil, or Cabot stain. The natural products should be
re-applied annually.

[TinMan1332]

[VHarris001]

I saw several posts that Thompsons was junk but I don't remember that any
alternatives were listed. What works?

The exterior ply I put on the pickup truck isn't separating, but water still
soaks into the edges and causes warping. I painted with good quality oil base
primer and paint, taking special care to cover the edges well. Then I used
silicone caulk along all edges, but it is still warping. Also, the surface of
the wood is cracking. Will a good sealer really stop this?

[VHarris001]

>No warping, no splinters, and the sucker is still holding up well
>(although its with the ex-wife now, sniff-sniff).

Rugeeky, I feel your pain!

Did you paint or seal the wood? How long has the trailer been in duty?

[Greg Fretwell]

I am coming up on 7 years with the plywood on my pontoon boat. When I
replaced the carpet this year the plywood was solid. It is MDO 3/4" and I
just soaked the edges with paint, put elastomeric roof coating on the
surfaces. This boat lives outside all the time and runs in salt water
regularly.

Greg

[VHarris001]

I've heard of seasoning PT, but haven't heard of drying it. I guess I thought
it was already dried. Can I get pre-dried PT? Is it expensive?

[VHarris001]

MDO, is that marine grade? Also, how durable is that elastomeric roof coating?
Now that I think about it, the scraping and rubbing of junk on the wood grinds
off the paint (even the Emron I used on the inside of the truck). I'll bet
that's how the water gets into the wood! Maybe all I need is a more durable
surface. Whadda ya think?

[rugeeky2?]

Anybody who does things right will hate me for this. . .

I spray painted the whole thing with bargain bin cans of black paint, used a
lotta cans, but it held up. The metal frame was showing some rust last I
saw.

Built that thing at least twelve years ago.

Cheers!

Joe

VHarris001 wrote in message
<19990510222533...@ng-fh1.aol.com>...

[..]

Yep, oil enamel on AC plywood is your best bet. Marine plywood is for
structural integrity (as for use in boat transoms) and not weather
resistant. Found this out when replacing boat seat platforms. Pressure
treated plywood I've seen is only 3-ply and is junk.
..

<dav...@myremarq.com> wrote in message
news:qxOZ2.5167$4S.3410562@WReNphoon3...

[Greg Fretwell]

You can get any kind of plywood you want "wolmanized" but the plant
operator told me the mere act of pressure treating the plywood may cause
it to warp, delaminate and bubble. Pressure treating plywood is only
insect protection, not weather proofing according to the guy who does it.
There are lots of plants around and they will pressure treat anything you
take them, to any CCA level but I was talked out of doing it for the
plywood on my boat by the Wolman rep.

Greg

>Pressure treated plywood I've seen is only 3-ply and is junk.

>...

[Walter Boomsma]

vharr...@aol.com (VHarris001) wrote:

>I'm putting sides on a utility trailer that gets hard daily use and is stored
>outside in the weather. Must have a smooth (A) and should have at least a B
>interior. I put exterior AB plywood sideboards on a pickup truck last year and
>it's also warping. The hardware store said the only thing I could do was dip
>it in a sealer (Thompsons, etc.) but didn't know anyone locally who could do
>that for me.

One other factor which I haven't seen mentioned yet... There's some
correlation between warping and span or run. (i.e., an 8' piece of
plywood fastened only at each end will warp more than if secured every
few feet.) If you have the option, more is better up to a point
(without over-building). You'll still want to do the sealing, etc.
mentioned, of course.
Walter
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