using interior ply on outside

[will guthrie]

Hello, I'm building the widget and have the walls up and about half of the 1/8 in. inner roof skin and spars on and was doin research on sealing and painting for when (or if) I ever get done. I used 1/4 interior birch for the outside walls and was planning on 1/8 interior birch for outside roof skin. I emailed the Rot doc asking some questions about CPES epoxy sealer and in response I could almost here him scream as he typed " DO NOT use the interior ply! Left me tremblin' and a bit demoralized. I had been apprehensive about using interior on outside in the first place but I seem to remember reading posts on this forum that that's what some have done and then sealed it well (like with CPES and then uniflex). Besides 1/8 exterior or marine can't be found here and is pricey from the likes of Boulter Ply. I've soaked an unfinished sample of the 1/4 interior birch in a cup of water for a couple of days without it delaminating (and will do the same with the 1/8 in) so I thought I was good to go. I'd be interested if others have used interior and how they fared. Thanks

[Miriam C.]

Interior and Exterior refer to the type of glue used to laminate the wood. Wood is wood. I used simple 1/8 Birch ply on the roof and 3/8" Baltic Birch on the outside. Spar varnish is the sealer because I intended to not use my credit card to order stuff. I think it will be fine, at least till I get the next one done.

[Tear Les]

Hello, I'm building the widget and have the walls up and about half of the 1/8 in. inner roof skin and spars on and was doin research on sealing and painting for when (or if) I ever get done. I used 1/4 interior birch for the outside walls and was planning on 1/8 interior birch for outside roof skin. I emailed the Rot doc asking some questions about CPES epoxy sealer and in response I could almost here him scream as he typed " DO NOT use the interior ply! Left me tremblin' and a bit demoralized. I had been apprehensive about using interior on outside in the first place but I seem to remember reading posts on this forum that that's what some have done and then sealed it well (like with CPES and then uniflex). Besides 1/8 exterior or marine can't be found here and is pricey from the likes of Boulter Ply. I've soaked an unfinished sample of the 1/4 interior birch in a cup of water for a couple of days without it delaminating (and will do the same with the 1/8 in) so I thought I was good to go. I'd be interested if others have used interior and how they fared. Thanks

Anyone familiar (as the Rot Doctor is) with plywood is going to tell you to NEVER use interior grade plywood where it will be exposed to weather. To do elsewise would be a disservice to the person you're trying to help. If you were just starting your trailer I'd tell you the same thing. Why? Because I don't want you to put as much work as it takes into building something like a trailer and then have a failure of material which is many times more likely with interior plywood as opposed to exterior rated ply.

So at this point you're the only one that can determine whether or not you can live with your choices. In other words, you're now gambling on your ability to seal and protect the interior grade ply versus the time you'll have to spend to repair or replace it if necessary at a later date. You absolutely MUST protect that interior ply from any moisture intrusion if you want to avoid damage (unless it's just a short term trailer).

Plywood doesn't get soaked to test it, it gets boiled. Take a piece of what you've got and drop it in boiling water for an hour and see how it holds up. Pretty good chance you won't use your trailer much in boiling water but you'd be surprised just how hot things get in the sun. This has been a standard test for years in the boatbuilding world where failure means you're swimming home rather than walking.

You CAN keep going from where you are. I'm not trying to tell you to rip things apart and start all over. I am saying that if you want a trailer that will last for years without constant repair you really need to make sure you've got that interior ply sealed against any water intrusion. Of course that's a darn good idea even with exterior ply.

I'd encapsulate the whole darn thing with a light fiberglass (6 oz) and epoxy and find a way to drill as few holes as possible (and seal the ones you do drill really well). That, however, is expensive but I can't think of another way to really feel comfortable with that interior plywood. Hopefully more folks will jump in here with some ideas.

The ultimate question I guess is how long you want this particular trailer to last? That will dictate a lot of your decision.

[Airspeed]

I dont know about the plywood you are using but after selling literally thousands and thousands of sheets of plywood and touring a couple plywood mills for my former job,I found out that most plywoods except marine grade plywood use the same glue,Im talking about AC,CCX,CDX,303-18,many oak and birch veneers,Luan,and alot of plywood marked "interior"
The glue they use is water resistant not waterproof,except marine grade and some pressure treated plywood.
You could probably write or call the mill that made yours and find out. I had many customers argue with me on this point until I put them on the phone with the mill.
You may be using a type of paneling that uses a glue similar to plain ol elmers and it wont take much moisture before it fails.
Find out who manufactured your plywood and ask them what type of glue they use. Good luck! Aaron

[will guthrie]

I got my birch ply from the local lumber store. It's a C-3 and the back side is pretty lousy looking but I looked pretty close for signs of voids and delamination and it seemed pretty good. It's pretty inexpensive stuff ($12/sheet). Assuming I do go ahead with it for the outside roof skin would CPES be the best thing for sealing? I do want to paint so a spar varnish is probably out of the question unless they make a good tinted version. Thanks

[Airspeed]

I used some cheap Luan 1/4" on the exterior of mine,I had to soak it in my hot tub to get it to bend around the front of my tear and it held together very well.No matter what type of plywood you use I would recomend that you seal both sides and all the edges before you use it.A little extra work now will save you a big headache later.
I dont think you will have any problems if you seal it up well and keep it sealed in the future,I would try to keep it in the garage during the winter or at least keep it covered with a good tarp. I have used "interior" plywood on a couple boats I built, I just saw one I built for a guy about ten years ago and it still looked good,he uses it for fishing every summer and it has held up really well, I used an oil base house paint on it originally and he has repainted it once in the ten years since I built it.
Aaron

[critter]

Hey Guys,
Dont make the same mistake I did and try to save a few bucks on plywood.The cheaper plywood is rougher which mean more sanding.What you save on plywood will be spent 4 times over on sandpaper,epoxie,primer,paint and elbow grease.It would have been worth a 100.00 a sheet for the amount of sanding I had to do and it still needs more!I was trying to build on the cheaper side but this is not the place to save money.JHMO!