Waterproofing plywood: Poly, epoxy....?

[Lew Hodgett]

DUH!!!

It's "Okoume", dummy.

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[Robatoy]

I once thought to buy a sheet to use as a bottom for a trash trailer.... Forgot the thickness I was quoted but it was over $ 300.00 So... no.

[Existential Angst]

I once thought to buy a sheet to use as a bottom for a trash trailer.... Forgot the thickness I was quoted but it was over $ 300.00

So... no.

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Heh, dat makes aluminum plate a no-brainer! Proly WITH anodization!!!

[Lew Hodgett]

------------------------------------- Not necessarily.

Some 2" urethane foam, some 17OZ double bias glass, and some laminating epoxy and you're done.

Slop on a coat of outdoor latex for UV protection.

Time for a beer.

Lew

[Jack Stein]

That seems to be a common imagination. However, last year I went to HD to buy a sheet of cheap $18 3/4" ply for the base of a wood rack. Instead, I found a pile of 3/4" ply on sale for $23 a sheet. The $18 stuff was junk, looked like cheap CDX, (which is supposed to be junk, regardless of where you buy it.) For just $5 a sheet more I passed on the cheap stuff. This was high quality, AB stuff with no voids. I bought it, but felt a little guilty putting this stuff as a base on a wood rack.

Also, my HD always has good lumber. The biggest problem I see is pricing. For example, they always sell 2x6's cheaper than the same grade of 1x6. At first, I thought it was a mistake, I even mentioned it to a sales guy once. It's been like this for at least a year or two, so it's on purpose.

As far as Oak lumber goes, my store sells only #1 select and it is perfect. You need a briefcase full of money to buy it, but it is perfect stuff if you are polluted with cash.

I never heard that before. I heard that about Sears though, Sears brand tools are cheaper versions of brand name tools. Could be true though, I usually buy off the internet for tax reasons.

[Jack Stein]

Yes, but the pool has a plastic liner so the pool water does not actually come in contact with ply. It is exposed to the weather though, and has extreme contact with water from normal spillage and leaks. It is an above ground, 24x32' pool, and the ply holds in the water.

I dunno, probably everything from pine to teak. My brother made a canoe out of orange crate wood 55 years ago, still works, and looks better than any canoe I've come across.

My impression as well. Also, what material is used for the inner layers has an effect. I've seen everything from solid wood planking to some sort of clayish looking wood putty stuff.

I've never seen or heard of a 4 layer ply?. All ply should be odd numbered?

Well, I've seen 3 layered ply with 1x4 solid wood planking as the core. It was stiff.

No, just the number of ply is not the whole enchilada.

Let us know what you find out.

[Swingman]

I doubt yours is 42" above grade ...

[J. Clarke]

I didn't see anybody asking questions, but there are some that are relevant.

When you say "outdoor use", that covers a lot of territory.

Are you going to be putting it on smooth pavemement, a well tended lawn, an irregular surface, a beach, rocks, or what?

How long will it be exposed? Is this something you're going to pull out once in a while and then put back, or is it going to stay outside permanently? If permanently, how long does it have to last? A day? A week? A month? A year? A decade? All eternity?

Does it need to be portable? If so, how portable? A five year old can lift it and carry it away? An adult? Two adults? Four adults? A lift-gate truck? A crane?

An ordinary sheet of Home Depot exterior plywood (it's much like other standard-graded exterior plywood--if you can break one piece of half- inch douglas-fir ply across you knee you can break all of them) without any protection should be able to survive outdoors in ground contact for several years--how many is going to depend on the environment--it will be longer in Death Valley than in the Everglades or Seattle but it's not something I'd try to quantify beyond "several years".

Pressure treated plywood (should be able to find that at Home Depot as well) will last longer obviously.

Painting it will prolong its life even more. As a general rule you want a pigmented exterior coating--clear polyurethane and other such are remarkable products but no clear coating is going to hold up as well under UV exposure outdoors as a pigmented one. Follow the manufacturer's recommendations for primer and whatnot.

Hardwood ply will be stronger but finding it with exterior glue may be problematical. Marine plywood is a very, very high grade, but it's not cheap and may be overkill for your needs.

Doing it "right" you would use ground-contact rated lumber for the actual ground contact, put some pressure treated stringers on top of that, and put your ply on top of the stringers, so it gets some air circulation and won't be wet all the time, and paint it all with a good grade of house paint (or you could go hog wild with Awlgrip or another

2K marine coating but that's serious overkill). That's going to be heavy and bulky though and for something that you want to be easily portable and that doesn't have to last very long would be excessive.

[Eric]

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.

==========================

Things to note

When you check the stiffness of plywood, especially with only three plies,there are two ply grains in one direction and only one ply grain perpendicular to that one.

"real plywood" Many so-called "plywoods", these days, are becoming chipboard coated with a ply of veneer on both sides. This is still called "plywood" as it still has"plies". = "garbage board with a covering"

[Larry Jaques]

Oh. I thought you lived in basement territory, and FFE -was- grade. My bad. So, the house was 42" above the caliche? ;)

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

[dpb]

On 7/9/2011 12:48 PM, Existential Angst wrote: ...

...

The coating _will_ fail at some point owing to the tendency of wood to move (and it will, though the coating will slow down moisture changes it will still change) and in the application there's going to be structural movement as well.

...

WalMart does that; afaik the BORGs don't; at least for the major distributors. "House" branded stuff is, of course, different.

As for lumber, Boise-Cascade, Georgia-Pacific, etc., are same products at wherever they are. As Swing says, what's different is what specific products they're carrying--much of the box stores' inventory is imported rather than the name brands so have to be observant. But, that's also becoming a trend elsewhere as well, sadly. If it has the APA (formerly American Plywood Assoc., now still use the APA initials but titled the Engineered Wood Assoc.) grade stamp you can infer the panel from wherever obtained will meet the standard for that grade (and that there won't be a great deal of difference in how much "better" one vs another is at that grade owing to competitive pricing pressure).

It would be useful likely to go review what those stamps on the sheets actually mean and the definitions behind them.

Specifically towards your objective might be--

Look for

--

[Existential Angst]

So you are saying, essentially, to make the base from scratch? Any more details, sources for doing something like this? Sounds interesting, economical, lightweight.

[Existential Angst]

Swingman did!! See my first response to him, but I'll address your specifics as well.

All of the above. Altho, for truly irregular surfaces, they would have to be evened out somewhat, bec altho my intended base is rigid enough for the "use stresses", that is sort of predicated on a uniform surface, altho that surface could be anything. Of course, if I really beefed up the base, the surface wouldn't need to be regular, but that would bump up the $$

Ideally, an eternity, permanent outside exposure. Practically, 10 years. An all aluminum/SS version would meet this, but it starts getting heavy, expensive.

Total weight, 100#, give or take. Thus it can be shoved around, with not much drama, but not by a 5 y.o.

Iow, if the base is 4 x 6, use mebbe three 6' ground-contact 2x4s (or thinner?), four or five 4' stringers across those (1x2?), and then the ply? I think the weight will come within range, and all those cross member+ply would certainly add to the rigidity. Food for thought.

[Puckdropper]

Jack Stein wrote in news:ivcnmj$hup$ [e-mail]:

*snip*

*snip*

You're probably looking at volume cost savings. The 2x6s can be used in a lot of construction, while the 1x6s are more of a finish thing. For woodworking, it works out for those with a little patience and a planer. Get the cheaper 2x6, resaw it and plane to 5/8".

Puckdropper

[Lew Hodgett]

------------------------------------- Find a fiberglass, resin distributor in your area.

Probably $200-$300 gets the job done for a 4' x 6' x 2" panel.

Lew

[Martin Eastburn]

I heard $75 the other day on TV.

So maybe you have no idea of today.

Mart> >>> Why not use marine ply. No voids in or out and the

[Lew Hodgett]

------------------------------------ Reminds me of the used car dealer in Tampa on TV advertising "Clean Northern Cars".

Lew

[Bill]

What's wrong with Florida cars? Salt? Heat?

[Morgans]

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.

[Existential Angst]

Definitely has potential, as a couple others have alluded to. A lot of homework, tho, ito shapes, costs, etc. Expediency is a bit of a priority now, but a lot of food fer thought for later.