Spar varnish on birch veneer plywood

[OzarkRay]

Things are going well on my build, and I'm to the point where I need to get the exterior finished before it's too cold. I'm building out under my carport, which is kind of limiting. I've used 3/4 birch plywood for the sides. I had been thinking of skinning it with aluminum, but I've decided to go ahead and just use marine-grade spar varnish on the wood. Is it necessary to use some kind of sealer on the bare wood before applying the varnish? I did a test portion on the inside and it seems like the varnish is getting sucked pretty deeply into the wood. Are those stain/seal products used for decking used for this purpose? I hadn't given a lot of thought to staining the birch, but if it's part of the process, I'd like it to be the color of Newcastle Ale Also, I'm insulating the thing, so I'm planning on using Thompson's water seal on the surfaces that will ultimately be covered by the insulation and thin interior wall panels. Is that the thing to do? I'm hoping that by doing that I will minimize the effect of condensation, which I'm assuming is going to happen regardless of what I do. Thanks for the help, I'm really glad to have found this resource!
Ray

[Esteban]

Have you considered sealing the plywood with epoxy? Then varnishing. Check out varnishing over epoxy to see how it's done.

[tony.latham]

I agree with Esteben on this. Get Raka's 1.5 gallon epoxy kit for a hundred bucks and give it a couple of coats. It'll darken the wood much like varnish. You still have to give it a couple of coats of good marine grade varnish to protect it from UV rays.

I bought the thin Raka since I was using it for a sealant and my birch soaked it up like water on the first coat.

TL

[oakinteriors1]

You can do what you want but this outside TV cabinet I built is in its second year and still looks new, coated with 4 coats of Helmsmans spar urethane...

They remove the shade-ezup during the winter..

tn_100_1492.JPG (85.69 KiB) Viewed 1323 times

[tony.latham]

Soak a piece of that exposed luan in some water for a couple of days and see what happens. Throw in another piece that's covered with your paint or varnish. Boat builders use what they call the boil test. A piece sealed in expoxy and thrown in the test bucket will be intact come spring.

That outdoor piece of furniture that Oak Ineriors posted is an outstanding piece of work. Gorgeous! It appears that it's made from cedar. Most plywoods with some way of intaking moisture will delaminate over time. As you are well aware, you've got an incredible amount of time in your build and a fair amount of money.

Tony