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Drying Wet Plywood in a Boat

12/11/17       
TomTom2017 Member

I was recommended I post on this site a question relating to drying marine plywood on a boat that I built; and which unfortunately had issues with the epoxy resin, allowing water ingress into the plywood.

I have attached a link to my "thread" on the other forum as it explains the issues better - but if this is inappropriate, I apologize and feel free to remove the post and I will edit it.

Basically, I have gutted the boat and sealed it under a plastic sheet with a dehumidifier and a fan inside. According to my moisture meter, the Moisture content of the plywood has dropped dramatically.

I am concerned that there may be unseen rot however - and my plan is to attempt to epoxy encapsulate the plywood when the moisture content is as low as I can get it. I am hoping that by doing this, if I can "seal" the plywood at a low moisture content, I can effectively protect against any potential for rot.

I am using Ampreg 21 epoxy resin and biaxial cloth.

Are there any treatments I can give the wood, that are compatible with the epoxy resin overcoat, that may help ensure there is no "residual rot"?

http://https://forums.bateau2.com/viewtopic.php?f=12&t=62817&sid=269b9394a4e58e2d4ad3ba78c7047f11

12/12/17       #2: Drying Wet Plywood in a Boat ...
Gene Wengert-WoodDoc

Rot or decay is only active and increases the amount of rot when the moisture is well over 22% MC. If you can seal the dry plywood and keep it sealed, there is no risk. The problem is keeping the seal perfect. If water gets beyond the seal, it is nearly impossible to get it out...the water is actually sealed in the wood.

12/13/17       #3: Drying Wet Plywood in a Boat ...
TomTom2017 Member

Gene thank you for your message. If there is a more appropriate place for this post, please let me know so I can move it there.

When I first started building epoxy-plywood stitch and glue boats, I was sold on the epoxies incredible ability to seal wood and make it innate.

However, the reality is that I have actually found this quite hard to achieve; and I now wonder if epoxy is more porous than I thought.

Take this current boat for instance. It is made with three plywood panels on each side - a hull panel, a lower side panel and an upper side panel that is epoxy glued to overload with the lower side panel by 6 inches. The outside is sheathed in biaxial fabric and epoxy, as was the inside. All screw holes were overdrilled, filled with thicked epoxy and then redrilled. Essentially "watertight"...

My bad epoxy/ delamination issues were limited to the inside of the hull and to the bottom and lower side panels. I know this because that was all sheathed in one day, with one certain batch of epoxy. All the rest was a different brand and done at a different time.

The "bad bits" have been completely cleaned, sanded and scraped to bare wood and left under the tarp with a dehumidifier inside.

My pin moisture meter is telling me ambient here (I am in the tropics) is about 16%; but my hull and lower side panels are now down to about 8-11% under the trap and dehumidifier. (This is similar to test pieces I put in my dehumidified room in the house).

When I tested the hull with a Pinless sensor before I started dehumidifying it, it was telling me that the relative humidity was much higher in the lower side panels and the bottom panels; now it is telling me it is the other way round. At this stage this is good news - the dehumidifier is working and I am drying the bottom panels and side panels which had water ingress.

However, it is also telling me that the upper side panel - which is still "sealed" with fiberglass and epoxy on both sides and has intact paint - and which was built in a place with a much lower humidity - has "quite" a high moisture content...

So I begin to ask myself - how is the moisture content in this "sealed" panel so high?

Does the boat living at a higher ambient moisture mean that eventually water does diffuse through the epoxy?

And if so, can sealing the rest with epoxy - as best that I can - ever really be trusted to stop it reaching that 22% mark that might cause rot?

12/15/17       #4: Drying Wet Plywood in a Boat ...
Bill Smith

Website: http://www.esf.edu/nekda

Unfortunately, the best way to avoid rot is to have initially used pressure treated marine grade plywood. As Gene noted, encapsulation is a great method until there is a leak, and then since moisture is trapped the situation can typically quickly turns bad.
One issue with pinless moisture meters is that they are sensitive to material density, so if you are measuring in a location that might have more epoxy resin, or wood than another a higher than true moisture reading may result.


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