Waterproofing plywood: Poly, epoxy....?

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Awl --
I'm looking to make a 4 ft x 6 ft base for some apparatus, out of 1/2" plywood, for outdoor use. It works well indoors, but I'm wondering if I can make it *truly* waterproof for outdoor use, with enough coats of poly, epoxy, or some clear plastic coating-type finishes I've seen.
Not that familiar with wood/coatings, beyond having done my floors with a water-based poly+catalyst, with mixed results.
Also, I know there's HD 1/2 plywood, which you can break across your knee, and there is real 1/2 ply, from a lumber yard. Are there even harder/stiffer grades? I'm not necessarily looking for furniture-grade plywood, but maybe that is indeed the stiffest. Cost, bang for the buck is a factor. Maybe other "engineered products"?
Appreciate all input.
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EA



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On 7/7/2011 3:52 PM, Existential Angst wrote:

A nice heavy coat of good old fashioned exterior paint will work wonders. After all, that what is on houses. Throw some sand in if you need traction control.
PS. i'd go 3/4". 1/2" is not good for much of anything.
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On Thu, 07 Jul 2011 16:16:48 -0500, Steve Barker

I'd go at least 5/8" if not 3/4", and he can order pressure treated exterior plywood from his lumberyard if they don't stock it. Spar varnish over that, for the flexibility if offers, should keep the platform solid for years and years. Be sure that the finish is well maintained.
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Start off with MDO maybe? With just a couple of coats of marine enamel, signs made that way can last 20-years+
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If your situation allows it, how about 5/4 boards like are typically used for decks? No coating needed. They're waterproof, strong, have rounded edges, and are small enough to be easy to manage while building. With some 2x under them, I've used them for a lawn tractor ramp and a 12' high play fort and they're going on 20 years.
----------------- Awl -- I'm looking to make a 4 ft x 6 ft base for some apparatus, out of 1/2" plywood, for outdoor use. It works well indoors, but I'm wondering if I can make it *truly* waterproof for outdoor use, with enough coats of poly, epoxy, or some clear plastic coating-type finishes I've seen. Not that familiar with wood/coatings, beyond having done my floors with a water-based poly+catalyst, with mixed results. Also, I know there's HD 1/2 plywood, which you can break across your knee, and there is real 1/2 ply, from a lumber yard. Are there even harder/stiffer grades? I'm not necessarily looking for furniture-grade plywood, but maybe that is indeed the stiffest. Cost, bang for the buck is a factor. Maybe other "engineered products"? Appreciate all input.
--
EA



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To make 1/2" plywood outdoor-tolerant, cover it with tarpaper or Tyvek (housewrap) and put siding on it. The intermediate layer keeps moisture in the siding from wicking into the plywood, and the siding takes the weather (rain, wind, sunlight, ivy, woodpeckers) until you get tired of how shabby it looks and replace it.
Paint-on-wood is less appealing than other siding choices.
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On Thu, 07 Jul 2011 16:52:24 -0400, Existential Angst wrote:

How about marine plywood?
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On Thu, 07 Jul 2011 16:52:24 -0400, Existential Angst wrote:

How about marine plywood?
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On Thu, 07 Jul 2011 23:16:47 +0000, Larry Blanchard wrote:

Hmmmm. Seems like everything I posted to any newsgroup yesterday showed up twice. I wonder if this will.
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On 7/7/2011 3:52 PM, Existential Angst wrote:

You simply do not give enough information for anyone to make an informed response that will stand the test of time.
What is the "base" being used for?
Is proposed plywood being installed horizontally (as with a subfloor) or vertically (as with siding)?
Will it be carrying weight?
If so, what is the sub-structure, and how much weight?
Will any part of it be directly exposed to the weather?
That said, there is really no reason to use indoor plywood outdoors, and to do so will ultimately be unsatisfactory regardless of the covering.
There are any number of types and dimensions of plywood/sheet goods made for "exterior" use, which will serve you better.
Also, the very same folks who supply HD and Lowes with sheet goods also supply "lumber yards" with same (and just because you buy sheet goods at a lumber yard doesn't mean it will be quality merchandise, particularly in this day and age), so don't rule out the big box stores for good buys on sheet goods. CAVEAT: you need to be familiar with the materials, and how they are made, to determine what is acceptable.
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First, thanks to all who responsed, and I'll answer the points raised here, point-for-point, and others.

A fitness-type "jungle jim", for hanging, swinging, etc.

Horizontally, and in contact with backyard-type stuff: grass, soil, sand, concrete, etc.

Bodyweight, from kids up to 500# adults. Which would sort of require that whatever surface is supporting this base be pretty level, uniform.

The units I've made already use "regular" 1/2 ply, which by itself is absolutely inadequate for the job. But because of "guy poles" (think guy wires, but rigid), and various gusset plates, there is surprisingly little stress (at least bending stress) on the ply. It serves more as a platform for screws/bolts, for the rigidifying members.. The net result is to keep the apparatus itself from tipping over.

All of it, year-round..

Well, this is the crux of my point. If indoor ply WERE sufficiently "encased" in poly, or some epoxy/plastic coating, COULD it become weather-proof? And at what cost?
I understand, now, from the other replies, that it would be better to start off with a more appropriate class of plywood. Really, the Q was sort of two part, asking about plywoods, and the effectiveness of coatings.
Wolmanized wood sounds intriguing. http://gawain.membrane.com/decks/lumber_and_pressure_treated_lumber/wolmanized_wood.html has a succinct description, and reflects what Jack was saying about wolmanized wood.
There are other solutions, which includes making the base out of aluminum plate, which I am actually in the process of doing, as a test. Not cheap, tho, altho when one factors in the whole finishing process required for wood (or perhaps the cost of wolmanized PT lumber -- haven't priced this out yet), mebbe alum plate won't be that far off, price-wise.

It's hard to imagine HD supplying ANYTHING of quality. My understanding is that even reputable power tool companies (Bosch et al) make HD versions. But indeed, caveat emptor, and the more one knows, the better one can buy. Easier said than done, however.
I am, however, a big fan of the Husky compressors that HD carries. Have bought two, VERY quiet units (for compressors), and so far so good.
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On 7/9/2011 12:48 PM, Existential Angst wrote:

For your stated purpose, and IME, your best be would be an APA rated exterior plywood coated with a water proofing "system" made specifically for the purpose, like this PolyCoat product :
http://www.bestmaterials.com/PDF_Files/Polydeck_400.pdf
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I saw these two products (Reader's Choice Top 100 Products list) in a remodeling mag this morning: UDF-21 http://www.greenbuildingsupply.com/utility/showProduct/?objectIDb65
http://www.ccaplaygroundsolution.com/ EPL
I wonder how the three compare. Have you used any of them? What's the difference in durability and/or longevity between the poly, urethane, and epoxy systems? (if you or anyone else knows)
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On 7/9/2011 5:21 PM, Larry Jaques wrote:

I used the Polydeck product about five years ago on a raised platform for AC condenser units that had to be at FF elevation to pass code (an Architect's design/plan), although I've seen the platform a couple of times since, as far as I'm concerned it is simply too early to tell.
That said, it still looked brand new about a year ago ... if you can extrapolate that to another five years, I would have to say that it was worthwhile and cost effective, and will most likely make it longer.
My problem with most exterior oil based 'weather resistant' coatings in this part of the country is the UV/sun just eats 'em alive. That does not appear to have the usual effect on this product thus far.
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Mine is a cheap prefab concrete slab they dropped on the ground when they installed the condenser. I saved a couple hundred by running the wiring myself, and lighting the unlit attic, too.

With the epoxy sealer and then 4 coats of other films, I sure hope not.
I've used Superdeck's opaque concrete stain and it still looks brand new five years later. http://goo.gl/htxaK Pressure wash, prime, and paint/stain. It goes on like slightly thinned latex paint and is very nice to work with.
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On 7/9/2011 9:58 PM, Larry Jaques wrote:

I doubt yours is 42" above grade ...
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Oh. I thought you lived in basement territory, and FFE -was- grade. My bad. So, the house was 42" above the caliche? ;)
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Why not use marine ply. No voids in or out and the glue is 100% waterproof.
This is wood rated to be in water when it is protected. Think boats.
It is used in flower boxes and such.
Martin
On 7/9/2011 5:21 PM, Larry Jaques wrote:

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"Martin Eastburn" wrote:

----------------------------------------- Price out a sheet of 3/4" Oakume (Marine ply).
Definitely overpriced for the application.
Lew
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DUH!!!
It's "Okoume", dummy. -------------------------------

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