How long does plywood need to dry before using?

I’m planning on replacing the plywood floor structure and 3/4” plywood support framing (stringers to but these are not plywood) in my 84 Century Coronado Cardel and someone mentioned to me that the marine grade plywood should be allowed to dry for like 2 months before using it. Not from rain wetness but because of the moisture inherent in the wood. They said especially since I plan on completely encapsulating it with epoxy.
I suppose it won’t hurt anything but I’d never heard that before. Any truth or logic to this?

 

Encapsulating wet wood is a sure recipe for rot.

How wet is the question.

The general answer is emc needs to be below 12%, less is preferred. This can be difficult to achieve in the tropics.

For saturated wood, I have found drying it to be like dancing with a temptress. What I mean is this...

I dried a wet boat in an plastic enclosure with heat and fans. The system would do a great job of removing surface moisture in the wood. I would check it and it would be 8-10% emc. Then I would shut down the heat and fans and the wood below the surface would get to equilibrium and drive emc back up to 17-20%. It took me awhile to realize drying wood takes a lot longer.

So, there are many factors to how long to dry something.

How wet to start.

Current conditions in the environment. (Coastal environments may never get you below 15% emc)

Air movement around the thing being dried. Air can stagnate in a bilge, for example.

New marine plywood should be very dry if stored in controlled climate and you don't leave it out for two months.

So, to answer you best needs a bit more detail.

 

If your plywood has moisture levels precluding the use of epoxy, you should switch suppliers, they didn't store it properly.

 

I agree. Unless it was left out in the rain, you should be able to used right away.

 

This would be brand new marine grade plywood, purchased from a supplier that keeps it all indoors. It will not have been subject to water. What the person told me is that there is inherent moisture in plywood and it needs time to dry. I’d never heard this before. It will be in my shop (Northern California) for about two weeks before I get to it anyway so I’m going to call that enough.

 

The person has experienced pressure treated ply wich was not kiln dried afterwards. Marine ply is not PT, so this problem doesn't exist.

 

Acclimation is done when you are concerned about movement of the plywood or any wood. For example, if you were going to build a wooden floor; you would acclimate the wood and allow it to reach equilibrium moisture content for the room it would be installed.

As Rumars said, totally not done for plywood that is going into a build.

Did you plan to seal the bottom of the plywood? At a minimum, a single coat of epoxy will avoid dry suck during bonding.

 

Once I get it all cut and fit I plan to epoxy all of the pieces on all sides and ends prior to epoxying it into the hull. I plan on using West System products throughout the project.
It’s pretty amazing to look at the original fiberglass job on the engine stringers and floor stringers. There are gaps where the original fiberglass didn’t even come close to encapsulating the original stringers. Definitely not what I expected to find in a Century. Maybe this boat was built on a Friday???

 

That is typical quality. The boat is 40 years old, so it has already outlived the original buyer's expectations. Epoxy is fine, but polyester will work. Polyester is cheaper and easier to work with. You will likely do a better job than the factory did, so it would easily last another 50 years.

 

polyester is not really waterproof, so there was no reason to achieve full encapsulation

if you want it waterproof; you'd need to sand it all and glass it with some decently thick glass with epoxy and seal any pinholes by neat coating

 

All those millions of owners of polyester boats will be upset to learn that. Its a marvel they haven't all sunk by now.

 

Not sure why you quoted me, when did I suggest it was a problem?

What does this have to do with boats sinking?

 

Polyester can be waterproof.
It just needs a thicker film thickness than epoxy.

 

Nothing is waterproof, there are only varying degrees of water resistance. For all practical purposes the typical polyester laminate is waterproof, the terrible workmanship on many old school production boats would lead someone to believe that it isn't though.

There are lot of 40+ year old polyester boats out there doing just fine despite the terrible build quality. Had they actually encased the wood in more than a coat of resin, then not drilled holes in it, the wood would have survived far better. Use the same build quality with epoxy and the wood would have failed just as quickly.

 

Class rule requires 12% moisture content before gluing. Builders claim 15% will do. Wood moisture meter are relatively cheap nowadays. From 10 to $15 on line.

Test conducted showed that epoxy has a higher penetration in wood than polyester. This appeared in PB article a long time ago. It is because epoxy is a "glue" while poly is not, To use poly as an encapsulation material for wood, a "mechanical bond" is needed. This is done by driving small nails in the wood with head partially protruding or partially sunk staplers, then a layer of mat with poly. VE is better than poly so, finish off with a layer of VE.