Epoxy over non-exterior plywood

[Watercamper]

I've searched and can't quite find the answer to my question. Let's assume that I'm going to use baltic birch on the roof of my TD. We know that it isn't rated for exterior use. So, If I coat the plywood with a coat of epoxy, have I now just made myself the equivalent of an exterior grade plywood? Same question for the 1/8" Luan that I see in the big box stores.

Secondly, can the epoxy be tinted and can UV stabilizers be added such that this is the only coating that would be necessary?

As you can tell, I know next to nothing about epoxy. Varnish, shellac, w/b shellac, latex, o/b & w/b urethanes and poly's, yes. Epoxy, no.

Thanks, Ken

[rowerwet]

epoxy encapsulated plywood becomes inert, as long as the encapsulation is complete, and doesn't get broken.
there are UV stabilized epoxies out there, I have no experience with them except to know they are more expensive than I would bother with. Even then I have seen warnings that the UV protection isn't forever.

[Jim Edgerly]

Since the exterior of my camper takes so much abuse from the weather, I used marine grade plywood on the exterior, 3 coats of epoxy resin, then 3 coats of a spar varnish to give the UV protection that the resin needs. Do it right the first time and you save yourself a lot of time and expense down the road.

[Tom Kurth]

Encapsulating non-ext. grade ply does NOT give you the same thing as ext. grade. What distinguishes the two is the glue that binds the plies together. If your encapsulation is imperfect from the start or is at any time breached, delamination is sure to eventually follow.

Best,
Tom

[Woodbutcher]

I have never used marine plywood in my builds. My belief is that if you seal everything real well, especially the edges you will be fine. My first trailer I used epoxy. Clear on the sides and black epoxy on the roof and hatch. I pay special attention to the area where the roof meets the sides and the bottom by the frame.
The second trailer I used the poly paint base below.


Here is a test panel that is 1/4" cabinet grade Mahogany. It has 2 coats of exterior polyurethane paint "base" only . No tint was added. I then left this piece out in the weather for over a year. It rained on it and the sun shined on it all summer. The show plow covered it all winter. The next Spring I found it and this is what it looked like. No attention was made to cover the edges or the back.



the back....

[tony.latham]

I used a considerable amount of Baltic Birch plywood in last summer's build. Everything on the exterior was sealed with epoxy before it was sheathed with .040 aluminum. I was curious about the delamination potential of the BB. Unfinished samples were thrown in a bucket of water. At the end of the build, the soaked samples were still intact.

On this build I have going, it'll be the same. Everything on the outside will be coated with epoxy.

Tony

[S. Heisley]

I used 1/8" Luan on the outside because it was what I could find and I wasn't building a "woody". Laying a damp paper towel on plywood for 10 minutes can cause some to delaminate. It did so with the Luan. Unless you put a layer of epoxy on the Luan as soon as you can, you will be sorry. I put a layer of epoxy on as soon as I had the shape cut...probably should have done it before I even cut it. Once the epoxy was on, I could work with the Luan without problems. I put yet another coat on after the trailer was together and then followed up with a 2-part marine epoxy and top-deck marine polyurethane paint. Yes, the marine epoxy primer came already tinted gray; but, I still wouldn't trust it enough to use it as the final coating.

Your time and money is worth a lot. Don't short change yourself by cutting corners. ...My 2 cents.

[rowerwet]

all my boats and my tear are built with luan underlayment, not waterproof or marine. a coat of paint is all I use to protect the wood in the boats, with epoxy and glass seams to protect the vulnerable edges of the plywood. my tear has painted canvas to cover it, three years later most of it outside, and the only area that needs work is the side that didn't get canvased.
Epoxy is an expensive way to go, but there are millions of boats out there built with non marine plywood sheathed in epoxy.
doing the boil test on a sample of plywood will tell you alot about how water resistant the plywood is. I build with Sureply, which has a waterproof glue, even though it is made to go under flooring.
Sureply is sold at Lowes and is american made by patriot timber, good stuff for $20 a sheet.

[Tom Kurth]

Further info I should have mentioned about ext. vs. int. ply: All plywood graded ext., MUST be manufactured with ext. rated glue; int. graded plywood MAY be made with weather resistant glue but not necessarily so. Thus, int. rated ply may hold up to weather well or not. Ext. rated should be good though none should be directly exposed to weather. I personally wouldn't trust anything made in China unless I have direct assurances from a reputable seller that it is good stuff. (The big-box stores are not to be trusted in my experience.)

Best,
Tom

[Roly Nelson]

13 years ago, I built my little 1/2 Nelson Woody with 1/4 inch Lowes Luan plywood. I began protecting it with 3 coats of spar varnish, not Marine varnish, and have applied a fresh coat every year since I built it. There is no delamination or other problems with this non-marine plywood, but all edges are covered with wooden members and show no signs of water problems. Since then, I have built 4 others which have 3 coats of Bear Paint, from Home Depot, and after many years, no problems what so ever. I guess marine plywood might be a better choice, but much more costly. As far as I am concerned, if properly maintained and sheltered out of the elements, this cheaper plywood works just fine. However, if your cheap plywood fails, I deny ever recommending it!
Roly, the li'l ol' woody builder in So Calif.

[Watercamper]

Thank you all for the wonderful info. Since my initial post, I joined the WoodenBoatPeople forum and posted my questions there. These guys use plywood for many of the their projects. Their environment is harsher than ours at times so I thought it would be good to get their thoughts.

For a TD, their opinion was that marine grade plywood was not really justified - with caveats of course. It's amazing where they use Baltic Birch. They are very careful to protect it with epoxy and cloth and the necessary UV protection if warranted. They were very specific that the cloth was needed. Just coating it with epoxy was not good enough in their opinion.

The thing I take home from them and you guys is that if protected and maintained, I could use almost anything . Roly, I find it interesting that you didn't even use cloth. One thing they did talk about in the boat forum was the differences in the plywood of the past compared with today. So for now, I'm going to let the desire for a lightweight design at an economical price drive the design and see what happens.