Moisture concern for stitch-and-glue?

Due to injury, my newly stitched boat sat in the garage for almost a year.  I live in a very humid area (outer Cape Cod) and can feel the fuzzy raised grain on the plywood.  I already glassed the interior, but now I'm wondering whether there is danger of the plywood rotting inside the epoxy if I glass the hull and encapsulate the wood when its moisture content is too high.  Am I worrying too much (my standard MO) or should I really wait until the wood can dry out, probably months from now (hate to do that after waiting so long to get back into building)?

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RE: Moisture concern for stitch-and-glue?

  I wouldn't worry too much.  Back about 10 years ago I was doing a restoration on a small cruiser. Replacing the decks with Douglas Fir plywood took longer than anticipated. I covered the decks with polyethylene sheeting and waited till spring to finish the sealing and glass work.  These decks were lamilated 1 1/8 inch thick and had the "fuzz" you mentioned due to being outside under an A-Frams cover all winter.  Well, I still own the boat and the decks are fine.

To get a nice appearance you might want to lay down a coat of resin and then sand it smooth before adding your second and third coats.  This will eliminate the Fuzz from showing in the finished product.

Good Luck,

Captain Pearl

RE: Moisture concern for stitch-and-glue?

Was your project ever actually wet while you were away from it?

Is the space in which you were / will be working under cover, protected from open exposure to the elements?

Marine ply is pretty tough stuff. The selection of grades of wood witjh which to make it, as well as the process of gluing veneers together (along with the glue used itself) go a long way to ensuring it will endure conditions that would see more common softwood ply turn to mush.

If your project has never actually been wet since the interior was glassed I seriously doubt the ply has sucked in water vapor sufficient to create conditions for rot were you to get back to work with finishing encapsulation.

Do you have electrical power in this space? Might you have access to a dehumidifier you could set up to lower the ambient humidity in there for a week or so? That'd go a long way to lowering the moisture content of the exposed wood before you get back to mixing epoxy for encapsulation.

And as for coating first, then sanding?

I differ here with Capt. Pearl; my experience tells me you'll have a more satisfactory result doing a very light sanding (de-whiskering is a more apt description) of the exposed ply veneer, followed with a thorough vacuuming inside and out before getting to the epoxy application, for a great deal less work. You feel fuzzy raised grain now, ponder for a moment how much tougher it'll be once epoxy's been applied then cured.

RE: Moisture concern for stitch-and-glue?

  The reason I mentioned coating first is that I found the top layer of ply, the top lamination, to be very, very soft after sitting out all winter. The boat was under an A-frame and the decks were covered. Probably condensation.  Had I tried to sand off the "fuzz" my fear was that I was going to take a lot of the top lamination with it.  The first coat of resin I applied soaked in like a sponge and firmed up the surface. 

After that I lightly sanded it (this is ten years or so ago).  My memory fails me on the grit I used but I'd assume it was 220 as I was just trying to knock down any roughness.  I used a 5" random oribtal sander.  After the surface was smoothed I typically apply an additional couple of coats of resin and then these decks were glassed with 4 oz cloth and finished.  The cloth is there just for abrasion not for strength.

Best Regards,


RE: Moisture concern for stitch-and-glue?

Thanks for the replies, spclark and Captain Pearl.

I'm not sure how to say whether my boat was "actually wet" or open to eposure to the elements...  It's not unusual to have >90% relative humidity here with 0% chance of rain.  Also, my garage does have power but cannot practically be dehumidified for a week, since we need to open the big doors frequently.  (No luxury of a dedicated shop.)

The surface of my plywood is also "very, very soft".  Does this change your thinking at all, spclark, or do you agree that I just worry too much?

Thanks, again.  I appreciate all the help I can get .


RE: Moisture concern for stitch-and-glue?

“Very, very soft” would bother me greatly. That’s an atypical condition for what I’d view as mechanically sound material onto which I’d be willing to move forward with encapsulation.

Might you have any scraps or leftover material with the same surface condition that you could use as material for testing? The idea being to proceed as you would on your hull’s exterior, glass and all, then see how well the plywood underneath reacts once the epoxy + glass layer(s) have fully cured then you try to separate them? It’s referred to as ‘destructive testing’ and really the only way to determine whether the combination has been properly assembled.

As for the small project in a large, uncontrolled space:

Treat it as one would if you need to heat your workspace but can’t or don’t want to heat a space vastly larger than needed for your project. Construct a lightweight frame from whatever’s at hand (wood, conduit maybe or PVC water pipe) then cover with polyethylene sheeting. Put your dehumidifier inside, running 24/7 set to 50% RH or lower, and keep the poly sheet closed as much as possible. You’ll pull water vapor out of the space inside your enclosure and, in time, also your hull exterior.

(Out of idle curiosity, where are you that you have to endure 90+% RH? Where I am if it’s above 75% and 80°F or above I’m disinclined to do anything requiring much physical labor!)

RE: Moisture concern for stitch-and-glue?

May be of help: hand-held moisture meters tested

Calculating Equilibrium Moisture Content of wood (on-line calculator) by which one can quickly determine that an extended exposure to 80% RH @ 80°F will bring wood to 20% EMC over time, while 50% RH at the same temp. allows the EMC to drop to 9%.

(Thin wood ought to be more responsive than thick to changes in RH over time.)

Article on the website of the company that makes one of those moisture meters tested about moisture content of wood for various purposes.

> 20% EMC seems to be where wood will rot, while 12% EMC is about the max. for working with epoxy

Were I in your position I'd want to assure myself that further efforts toward finishing that craft would be worthwhile (however well-intentioned!) and that the result would be both safe to use as well as long-lasting and a source of pride.

RE: Moisture concern for stitch-and-glue?

To satisfy your curiosity:  I mentioned in the original post that I live on outer Cape Cod (Massachusetts).  The part of the peninsula where I live is < 3 miles wide between Cape Cod Bay and the Atlantic, so, yes, we get a fair bit of humidity :-) and not only when it is hot weather, but, more or less, year 'round.

Thanks again for all the information! 

RE: Moisture concern for stitch-and-glue?

   It's nice on the outer Cape.  Haven't been out there in a few decades but spent a lot of time there with my parents as a kid.

One final comment regarding your project.  I think that we may be over thinking this a bit.  You certainly aren't going to glue up soggy wood I'm sure that your project is reasonably dry.  Right?  Dry to the touch.

You're not crossing oceans here and we're not building a Saturn V rocket.  CLC provides good Okume BS 1088 Standard Ply.  It's made with waterproof/boilproof glues. It's not been sitting out getting rained on, it's been in the garage and due to humidity the grain is raised.  That's really all there is to it.

Glue her up, finish her and get out there and enjoy your boat.



RE: Moisture concern for stitch-and-glue?

That's exactly what I decided to do.  She's sealed and light sanding is underway.  Thanks, Captain!  

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