How to make plywood waterproof?


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I am thinking of building a camping stuff organization system for the bed of my truck out of plywood (metal would be better, but I know how to make stuff out of plywood!). When driving it would be under cover (tonneau) but when camped it would be exposed to the elements, i.e. it could get rained upon fairly steadily for a day or so at a time.

How would you guys make it waterproof? Is there some kind of paint/goop that I can put on? I think plywood boats usually have fiberglass layer(s) over them (seems like overkill)? Any suggestions? (I don't care about appearance for this application). My understanding is that marine grade plywood is not actually waterproof (plus, who knows where I'd get it in a small town in the middle of the desert!).

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7 minutes ago, rodger. said:

Our local HD sells pressure treated plywood. Would that be adequate?

I don't know for a fact, but suspect that while it wouldn't rot it would swell, so it wouldn't be good for having doors and maybe drawers.

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i've given some thought to working with ply outdoors. I was thinking finishing the piece in epoxy would probably work well. For my outdoor (solid wood) shave horse the advice was to epoxy the end grain of the feet that touches the ground. 

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Steve is on the right track. Build it like a house, so it sheds water, and paint it thoroughly.

 

My grandfather used to keep a "dog box" in the back of his pickup, for going 'coon hunting. He laid a sheet of aluminum over the top, folded over the edges, and tacked it down about 1.5" below the edge. Keeping water from standing on top, and keeping the box from sitting in a puddle, goes a long way. Even without paint.

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If you want your outdoor plywood furniture to last, you should use marine grade ply. It worth the extra bucks. There's two different ways to bring up the topic.

First method, you want to seal the wood. You need an extra strong finish and no leak at all for the wood to keep its initial moisture level. IMHO paint is not the proper way to go, because it only seals by numbers of coats and you need to maintain the paint each year with some new layers. I prefer UV resistant crystal epoxy. It really seals the wood with a single coat and is lifetime. You can polish the surface several times if you get some unwanted scratches. On the other hand, this is a glossy plastic look some people would not like.

Second method, you want to allow the wood to breath. Even if plywood is a stable material, it can last a lot with a light breathy finish and an annual care. You can use hard oil (like teak oil). It's more natural to the touch and it can stand rain and moisture. Apply in at least 3 coats. I like to warm up the oil to 60°C (140°F) for the first applications. It goes deeply into the grain and will protect the wood definitely. The first coat can be heavy, but don't over charge. Do some tests to figure out the right amount. After winter time, just do a light sanding (P240) and apply a new coat of hard oil, and it will last for the year. This is a really fast process and your furniture will look as new.

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8 hours ago, GeneT said:

You should consider MDO.  Will probably need to purchase from a plywood supplier.

https://www.andersonplywood.com/395/mdo-medium-density-overlay/

+1 on the MDO. I had to replace the jambs on my garage door several years ago & used MDO ply. I sealed the bottom couple of inches with epoxy where it touches the concrete. It's on a north elevation where it never gets any sun & the concrete is almost always wet in winter. So far it looks as good as new.

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MDO is awesome stuff ! Around here they call it sign painters board. The MDF faces are made with lots of exterior resin and paints up beautifully. It's heavier than exterior ply & costs more but it's flatter and so easy to finish. Do pay attention to sealing up edges and keep them slightly elevated from sitting in water and it lasts for years in the weather. 

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1 hour ago, wdwerker said:

Do pay attention to sealing up edges and keep them slightly elevated from sitting in water and it lasts for years in the weather

Yes, I forgot to mention that I painted epoxy on all the edges that are exposed to weather as well. 

MDO also seem quite a bit stronger than ordinary ply.

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3 hours ago, Ronn W said:

Would MDO be a better product ot use for Table saw sleds etc.  Even Baltic pirch plywood is not so straight sometimes.

I've only seen MDO in 3/4" & that's a bit heavy for a sled. I suppose they make it thinner though.

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The 2 huge doorways we're making at work, the internal structure is all MDO with some mahogany dimensional lumber, and will be encased in mahogany solids for the painting surface.  It's really nifty stuff. 

1 hour ago, wdwerker said:

They might make it in 1/2" but I've never seen it.

We have a couple pallets of it in the shop.  Doing a quick google search, it appears it's available at Menards.....

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Laminate is made of high pressure phenolic papers. It's usually 1/32 or less thick. The MDO faces are definitely thicker than that and they have a much higher level of resin in them & they aren't as hard & dense. MDF is kind of like a thicker sheet of paper. So maybe the MDO faces are between the two. 

There is also HDO which has smoother & harder faces. Some places have exterior MDF or HDF which paints well but I have no idea how long it lasts. 

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