Waterproofing plywood: Poly, epoxy....?

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.

Reply to
Existential Angst
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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.

Reply to
Steve Barker

Start off with MDO maybe? With just a couple of coats of marine enamel, signs made that way can last 20-years+

Reply to
Robatoy

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.

Reply to
whit3rd

How about marine plywood?

Reply to
Larry Blanchard

How about marine plywood?

Reply to
Larry Blanchard

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.

-- Happiness lies in the joy of achievement and the thrill of creative effort. -- Franklin D. Roosevelt

Reply to
Larry Jaques

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.

Reply to
Joe

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.

Reply to
Swingman

Wood is pretty much waterproof already, glue not so much. Outdoor plywood uses water resistant glue, so it won't come apart so easily. Marine plywood uses a good quality water proof glue and has no gaps in the veneers for water to hide in. Marine plywood is rather pricy and probably not needed for "some apparatus", whatever that is. I have bought 3/4 wolmanized plywood at my home depot, and used it for the walls on my swimming pool. It has held up fine w/o any finish. If I were making a "base for some apparatus" I'd likely look into the 3/4" wolmanized stuff. I would not likely use 1/2 inch for a base.

The nice thing about the wolmanized stuff is there is no need to try to protect it with finish. You can finish it for looks,with paint or stain, but that is up to you. Actually, I guess that's true with any quality outdoor plywood.

All plywood is "real".

Yeah, 3/4" is stiffer, but really, there are a myriad of grades of plywood, and all sorts of different cores for different purposes. furniture grade is not what you are looking for outdoor use for an "apparatus" base.

. Cost, bang for the buck is a factor.

My home depot often has great quality plywood at great prices, but not all the time. Right now they have some sort of "heat treated" stuff on sale I've never seen before. It's red-ish stuff, I think outdoor. I'd look into it if I were building an outdoor base for an apparatus.

Reply to
Jack Stein

Hmmmm. Seems like everything I posted to any newsgroup yesterday showed up twice. I wonder if this will.

Reply to
Larry Blanchard

"...order pressure treated exterior plywood from his lumberyard if they don't stock it. Spar varnish over that, ..."

The first part "pressure-treated" plywood made sense - though NOT for Ground Contact - but the "Spar Varnish" part may prove problematic. As I understand it PT Wood is, essentially "wet" at the point of sale. I've been advised that coating/painting it too soon - before it has a chance to age and dry out - can cause it to rot.

I have NEVER seen a successful application of any plywood in contact with the ground. Though you did not indicate ground contact in the OP, thought to mention it just in case.

Oil-based primer, at least two coats with 24 hours to dry between and a similar number of coats of exterior enamel (latex is fine) is how I did my replacement garage door section. But a year of sprinklers wetting it (Florida sunshine and humidity) started the rot again.

I built a couple of barn doors with hardwood rails and stiles and a center web made of 1/2" OSB painted the same way and they've done very well for three years now.

If you're finished project is laid horizontally above earth or concrete - I would suggest you create openings to provide good air- flow beneath the structure regardless the finish.

Reply to
Hoosierpopi

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.

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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.

Reply to
Existential Angst

You put this wood IN the pool, to actually contain the water mass?? Details, please!!

And I guess this is not that unusual, given wooden-hulled boats, and the roof-top water tanks you see in NYC.

What types of wood are used in wood boats, water tanks?

If I

Weight is an issue, and it appears that structurally, I can get away with

1/2". 3/4" is of course an option, perhaps "heavier duty" versions".

I was under the impression that for a given thickness, the more layers in the ply, the stiffer it was. However, I was actually testing pieces of ply I have laying around, and a

3-layer ply was signficantly stiffer than a piece of 4 layer ply, both 1/2" And just now I roughly tested two pieces of 3/4 ply, one with *eleven layers*, the other with 5, and the 5 layer piece feels a little more rigid!!

So I guess that theory is not reliably true.

I'll check it out.

Reply to
Existential Angst

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 :

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Reply to
Swingman

remodeling mag this morning: UDF-21

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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)

-- One word frees us of all the weight and pain of life: That word is love. -- Sophocles

Reply to
Larry Jaques

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.

Reply to
Swingman

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.

Mart> >

Reply to
Martin Eastburn

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.

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Pressure wash, prime, and paint/stain. It goes on like slightly thinned latex paint and is very nice to work with.

-- One word frees us of all the weight and pain of life: That word is love. -- Sophocles

Reply to
Larry Jaques

----------------------------------------- Price out a sheet of 3/4" Oakume (Marine ply).

Definitely overpriced for the application.

Lew

Reply to
Lew Hodgett

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