sealing interior grade plywood

[LDK]

I'm building a solid wall TD out of 3/4" interior grade oak plywood. Would CPES be a good choice for the outside and edges and maybe spar urethane on the inside? Its also going to have an aluminum skin. My main purpose is to make it as waterproof as possible since it is interior grade plywood. The roof will have 1/4" luan and aluminum too. Also is there anything besides CPES thats about as good that I can get locally. Just a rookie asking for advice.

[GPW]

We've used 75% MineralSpirits(paint thinner) +25% Cheap polyurethane as a great sealer ... several coats works well ... Inexpensive if you shop around...
Those who tried CPES like it too ...

[doug hodder]

Any trailer that I do with an aluminum skin on it, I always seal the exterior substrate, as an "ounce of prevention". I use regular epoxy, not a fan of CPES, but since it's not going to be exposed to all the elements, something other than epoxy might work fine for you.

I had an original Kit from the late 40's with no sealer on the wood and it held up fine...well sorta, the aluminum roof was compromised and did leak but the walls were just fine at 40 years. Regardless of what you use as a sealer....make sure that your bottom edges (especially) and any exposed end cut in the ply is very well sealed. Doug

[LDK]

We've used 75% MineralSpirits(paint thinner) +25% Cheap polyurethane as a great sealer ... several coats works well ... Inexpensive if you shop around...
Those who tried CPES like it too ...

I think I might try your 75%-25% sealer on the outside and edges and then use some watered down TB2 and go back over the edges. Question, will the TB2 stick to your homemade sealer?

[GPW]

Maybe try putting on the TB2 first , then the mixture ,which will waterproof every where the TB2 didn't get ...

[LDK]

I wonder if spar urethane would be better than polyurethane in your 75%-25% sealer mixture. It's suppose to expand and contract because its made for outdoor applications.

[cracker39]

On the Squidget, I used spar eurothane as a sealer before priming and painting. In a little over a year I had a couple of places low on the front and rear walls where water had gotten into the plywood. I stripped down the entire surface, replaced a couple of plywood pieces and resealed with epoxy resin before priming and painting again. After 3 years and a lot of road traveled, the new owner reported only one small blistering near the rear cargo door opening where water must have gotten in. I failed to back seal areas around openings, a mistake I won't make again.

[Cliffmeister2000]

My weekender is made with sandwich construction, and the outer skin is 1/4 inch interior big box plywood. It is sealed with 3 coats of CPES, then painted with Uniflex 255. It appears (after 2 years) to be holding up just fine.

However, we do not have the rain found in other parts of the world, and I do store it in my garage.

One thing I found with CPES, it adds significant strength to the plywood. Cutting the ply after applying CPES, I used a blade rated for metal rather than for wood.

[cracker39]

It is sealed with 3 coats of CPES, then painted with Uniflex 255. It appears (after 2 years) to be holding up just fine.

Have you calculated the cost per square foot per coat for using CPES? I know that stuff ain't cheap.

[Cliffmeister2000]

It is sealed with 3 coats of CPES, then painted with Uniflex 255. It appears (after 2 years) to be holding up just fine.

Have you calculated the cost per square foot per coat for using CPES? I know that stuff ain't cheap.

No, but I think I spent $300 on CPES and Uniflex 255. I have 3 coats of each, and some left over of each.

[LDK]

My weekender is made with sandwich construction, and the outer skin is 1/4 inch interior big box plywood. It is sealed with 3 coats of CPES, then painted with Uniflex 255. It appears (after 2 years) to be holding up just fine.

However, we do not have the rain found in other parts of the world, and I do store it in my garage.

One thing I found with CPES, it adds significant strength to the plywood. Cutting the ply after applying CPES, I used a blade rated for metal rather than for wood.

I think I'm gonna try GPWs mixture of 75% mineral spirits and 25% polyurethane or maybe try spar urethane in its place.
Thinning it will help it to soak in deeper plus I'll apply several coats.
I'll put on several coats that way and then maybe 1 coat full strength

[GPW]

LDK , I always tell everybody to try a test sample First , on a piece of scrap ... That takes away any doubt about the effectiveness...

I always use that cheap polyurethane ,the stuff you can find in a more Thrifty store..(CHEAP). That old nasty stuff that dries slick and seems to never dry ... Mixed with the solvent it works fine in this application ... but you're welcome to try using any poly you want .... it's ALL about sinking deep into the wood , encapsulating the fibers with "plastic" , rendering the wood waterproof...
I found this mixture years ago from a man named Chuck Felton ... who makes planes (models) out of common cardboard ... totally impractical in our damp climate , the mixture made it Practical ... I made a bunch over the years , never had any soggy planes ... well proven , to me anyway ...
For further reading on the subject , you may want to Google Chuck Felton and see what he's done... http://home.earthlink.net/~charlesfelton/