BoatBanter.com - View Single Post - Marine grade? or salt-treated plywood?

mmc

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On Wed, 21 Jul 2010 18:27:24 -0400, "mmc" wrote:

"Califbill" wrote in message
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On Tue, 20 Jul 2010 12:10:34 -0700 (PDT), Tim
wrote:

I was wondering abotu using salt-treated plywood on the Marquis
deck.
I'm not a feared of paying for marine grade, but Jimmy the carpenter
told me that the newer salt treated woods are about as worthy as
marine grade at a fraction of the cost. So, what's the opinion?

I'm even wondering abotu using hybrid decking like you'd use on a
porch. synthetic mylar etc mixed with wood. The stuff is weather
resistant as all get out!

I looked at everything when I decked my pontoon boat 20 years ago.
We
had a Wolman plant here at the time and they offered to pressure
treat
any plywood I would bring them but the guy who ran the plant said PT
really only protects it from bugs. If water is your issue, save your
money. I ended up with MDO plywood (one side) and it is still going
strong after 20 years in salt water. That is basically 7 layer
exterior grade plywood with one MDF layer. You would probably have
the
same luck with BC
The main thing is to be sure everything is sealed before you put it
in
and seal all penetrations. The only bad spot I have found in my
current rehab is from an unsealed penetration.
Cut all of your pieces to size, paint them on all 6 sides with 2
coats then install them. Paint it again to cover the screws. Bristol
Palin will be running for president before that plywood goes bad.

I worked on a sailboat once that had a PT plywood sole and where it
met, the aluminium mast was badly pitted.
I thought it may have been caused by the cyanic acid?

Nope, the copper is not friendly to aluminum in a wet inviroment.
Is ther copper in pressure treated plywood?

Yup. Used to be copper arsenate (CCA). Now it is a copper only
solution.
http://www.strongtie.com/productuse/PTWoodFAQs.html

Ah galvanic action. Good reason not to butt up to, bolt or clamp to
aluminum.
Thanks.

It may be somewhat of a concern but not a huge problem. These aluminum
brackets have been on my .8 CCA PT dock for over 20 years

http://gfretwell.com/ftp/aluminum%20and%20PT.jpg

Still look pretty solid!