Exposing Ply to the Elements


New User
Ever use quality interior plywood, say baltic birch, for an external (meaning: full exposure to the elements) application and make it work? That's what this thread is about.

I'm tasked with building a "tiny library". I can save a lot of time simply sizing a sheet of quality plywood rather than edge-bonding cedar or PT boards. So I'm exploring this possibility.

Delamination is my concern... caused mostly from water penetration along the sheet edges. I figure some TB3-glued solid PT edge strips will fix the most egregious point of water infiltration, while the flats will get coats of poly prior to paint, cladding, or whatever. I know purpose-designed water-resistant exterior-grade ply exists. But that stuff is hurty-expensive, or in the case of PT seems of mediocre quality.

Thoughts? Suggestions?


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I have used it on a half pipe for skateboarding. If you coat with Spar Urethane, it will last 3-4 years when exposed to the elements.
Perhaps the time is longer for other applications as the surface of a skateramp does get physical abuse. Typically the pieces on the flat bottom tend to go first as the water stays between the two layers of ply. The transition maintains for much longer. UV damage also seems to play into the degradation.

Mike Davis

Corporate Member
I would not use interior plywood for an outdoor project.

A-C exterior plywood or Marine plywood will last ten years or more if properly primed and painted.


Drum sand the edges smooth and apply a marine epoxy. You have to harden that smooshy stuff in between the plys to prevent fast water infiltration and delam from expansion. You may be able to use the marine epoxy directly as the bond layer for any edge strips you may want to attach to the edges. Though I havent tried this-normally I would continue on top of the epoxy layer for attach with something like Titebond III.



Corporate Member
Why not use the right materials for the job?

I made a half a dozen birdhouses from pre-finished domestic furniture grade plywood off-cuts, rather than put it in the dumpster. They lasted exactly a year, before delaminating and falling apart.

We had some Bluebird fatalities in the process, which was very unfortunate.

Thing to remember is any wood finish slows down moisture transfer in or out, however they are all permeable. Plywood used for interior uses glue which is not water proof and the layers orientation is such that there are stresses between the layers. The glue holds the entire structure in place. Even if you seal the ends, moisture penetration through your finish along the surface, relieves some of the internal stresses, which first means warping and ends in delamination.

Good luck.
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Senior User
The DIY teardrop trailer community has a lot of experience using plywood in ways you wouldn’t normally use it. Here’s a link

to an approach to finishing it they call “the mix” that has worked well and has a lot of experience behind it. I haven’t tried it personally.

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Phil S

Board of Directors, Events Director
Phil Soper
Staff member
Corporate Member
I would use MDO ply. It is made for exterior use, commonly used for signs


Senior User
What about using Advantek, a 5/8" sheet goods used for sub-flooring in the construction industry. Seems to hold up well to the whether, but may be cost prohibitive. It is available from the big box stores.

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