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Old 29-02-2012, 07:25   #1
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Should Plywood Boats be Sealed Both Sides ?

hi there,
i recently purchased a 34 ft plywood catamaran that was constructed using stitch and staple/epoxy glue method.the whole exterior has been finished with fibreglass mesh and epoxy resin.the guy that built the boat has not sealed any of the plywood,stringers or any of the internal woodwork.he is adiment that one side of all the wood should be left unsealed so it can breath and if it does get wet so it can dry out. other people i have talked to with experience have said exactly the opposite and reccomend i seal the entire inside with a brushed/rolled layer of epoxy resin. there is a lot of water in the air on boats and leaving wood exposed does seem concerning.
any advice about this would be greatly appreciated.
thks
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Old 29-02-2012, 07:36   #2
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Re: should plywood boats be sealed both sides ?

Conventional wisdom is to encapsulate the plywood with epoxy resin taking special care to fully seal all edges. So yes, I suggest both sides plus all edges be coat with a 100% solids epoxy resin.

Not sure where you are in the world so West System or International are a good recommendations as they available almost everywhere.

If in Oz, the Bote Cote is my choice of epoxy - have used 100+ litres of the stuff.
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Old 29-02-2012, 10:10   #3
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Re: should plywood boats be sealed both sides ?

I used to think ply should be totally encapsulated but like you I have received conflicting advice.
I recently bought a 25 year old ply/glass trimaran. The boat needed a lot of work but the ply, which was left bare on the inside, was in very good condition, and if it ain't broke after 25 years I ain't fixing it.
One thing I have been told is that for epoxy to be really effective it needs to have fiberglass cloth in the installation, not just bare resin.
I am not an expert on the subject.
Wiser minds?
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Old 29-02-2012, 10:34   #4
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Re: should plywood boats be sealed both sides ?

I have seen boats with fiberglass on only one side, (effectively what you have is a wooden boat with a good paint job). If however all sides of the wood are completely sealed with resin, what you have is a plywood cored fiberglass boat.

Both can rot with water intrusion. I am redoing an interior right now that was done one sided with fiberglass. the water pooled inside, and rotted between the wood fiberglass interface.

In short both systems can fail, but my preference is to completely coat the wood. In another boat I recently had to replace the bilge decking, (it was rotted), I dropped the completed wood piece in a large pan of resin and let it soak until completely saturated, then covered with glass mat, and when cured, glassed it into place,...It will likely outlast the boat.

I had to remove a simularly treated piece from a 1960 hull, it wasn't rotted, it was in new condition, but in the way of a remodel. I found no tool or combination of tools able to remove it. I tried saws, blades, hammer, chisel, sledgehammer. It is still in place to this day, I finally gave up, and had to work around it.
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Old 29-02-2012, 10:48   #5
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Re: should plywood boats be sealed both sides ?

Quote:
Originally Posted by capn_billl View Post
I have seen boats with fiberglass on only one side, (effectively what you have is a wooden boat with a good paint job). If however all sides of the wood are completely sealed with resin, what you have is a plywood cored fiberglass boat.
----
An exterior layer(s) of fiberglass offers much more protection than just a coat of paint.
I think you would have to encapsulate the wood with fiberglass and resin, not just resin, to have a cored fiberglass boat.
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Old 29-02-2012, 10:58   #6
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Re: should plywood boats be sealed both sides ?

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Originally Posted by FSMike View Post
An exterior layer(s) of fiberglass offers much more protection than just a coat of paint.
I think you would have to encapsulate the wood with fiberglass and resin, not just resin, to have a cored fiberglass boat.

True; perhaps I should have made that more clear.
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Old 29-02-2012, 11:53   #7
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Re: should plywood boats be sealed both sides ?

It's a boat, it's wood, it WILL get wet eventually. Would you rather have it dry out, or dry rot? Leave it unsealed.
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Old 29-02-2012, 11:56   #8
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Re: should plywood boats be sealed both sides ?

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It's a boat, it's wood, it WILL get wet eventually. Would you rather have it dry out, or dry rot? Leave it unsealed.
I think the same argument could be made for sealing it. A barrier layer would be a great way to prevent dry rot.
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Old 29-02-2012, 12:15   #9
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Re: should plywood boats be sealed both sides ?

the quality of the plywood used will determine its resistance to rot and delamination,also the climate that the boat is kept in will have a direct bearing on the vessels rate of deterioation internally.

personally having had a cold moulded ply boat that was glassed inside and out,and that after 20 years still was as sound as the day it was finished,i would advocate at least epoxy inpregnation on the inside with glass cloth to the waterline/bilge area.
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Old 29-02-2012, 12:18   #10
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Re: should plywood boats be sealed both sides ?

Quote:
Originally Posted by tager View Post
It's a boat, it's wood, it WILL get wet eventually. Would you rather have it dry out, or dry rot? Leave it unsealed.
Just one problem with your advice... Plywood doesn't dry out never if it get's wet btw the veneers so your advice is, not worthless, but dangerous...
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Old 29-02-2012, 12:31   #11
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Re: should plywood boats be sealed both sides ?

Why would you barrier coat the boat's INTERIOR? If you have water in there, you have big problems! Better to let the wood breathe imho.
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Old 29-02-2012, 12:47   #12
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Re: should plywood boats be sealed both sides ?

I have seen both examples of construction,, and have seen the rot in both cases,,so i think one answer does not cover all cases here,,too many variables..
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Old 29-02-2012, 12:48   #13
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Re: should plywood boats be sealed both sides ?

Don't think wood as planks and wood as plywood are the same. Inside plywood is allready several barrier coats and the effectively resist all effort to dry the inner layers out. Thats why it's important to seal all edges with extreme care. The skins of ply could theoreticaly be without coating, but not even the best marine grade plywood isn't flawless and one tiny hole in btw the veneers lets water to penetrate and when it's inside it wont dry... Never... So to be on the safe side allways coat your plywood from both sides..
BR Teddy
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Old 29-02-2012, 13:25   #14
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Re: Should Plywood Boats be Sealed Both Sides ?

Wood whether it be in the form of ply or planking absorbs moisture at a set rate depending on the humidity etc. Wood sheathed on one side absorbs at a different rate to its open side so effectively one side is contracting or expanding ie MOVING constantly.

Rot will only occur where there is freshwater.

Personally i always seal both sides. BUT i would use an Epoxy preservative rather than just resin......

Cheers, Frank
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Old 29-02-2012, 18:27   #15
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Re: Should Plywood Boats be Sealed Both Sides ?

thanks for all your advice everybody.its a difficult topic with so many different opinions.
i am in asia,between malaysia and thailand.the plywood is locally made and the guy that built the boat boiled a piece of it for 2 hours and it held up fine.i have had a piece sitting outside,fully exposed,unsealed for 3 months now getting drenched and then dried in harsh humid heat and it is still in very good condition.the ends have not opened up, it just looks a bit dull.i think what i will do is seal any exposed ends with epoxy and as i will be living on board keep a close eye on it.
thanks again for your comments
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