Waterproofing Plywood


B

beiland

Well-Known Member
I'm building a layout in a shed external to my house in the hot, humid state of FL. While I have the shed under a carport and insulated well, it does seem to remain cool for the most part. But I do not intend to run the AC full time, so it will experience temp and humidity swings.

Many of my decks/shelfs will be good grade, multiply layered 3/4" plywood, which I intend to try and 'seal up' as much as possible against moisture absorption, and the subsequent warping that can occur.

My first thoughts turned to paints,...either oil or water based. But as I thought more about the situation I came to believe that the paints for the most part just coat the exterior of the plywood. They don't really penetrate the wood to seal it up.
 
Deck Type Sealers


I had some Thompson water sealer product setting around I had acquired from some estate sales. I looked up info on the product, and I was NOT very impressed with its capabilities for protecting outdoor decking. Seems as though many folks were unhappy with its performance.


But that said, I do NOT intend to utilize it in an outdoor situation,...where UV can be a big problem. I ran a few test samples on a portion of my outdoor wood deck, and some wood shelves I had made of plywood. I did note that it seemed to soak into the wood much more thoroughly than paint would have. That would seem to indicate a 'sealing function'?


Has anyone had experiences with sealing up plywood with deck coatings?


Can they subsequently be painted over??
 
Waterproofing Plywood. Sealing Plywood for longer life.


I have to clarify that I went to a different search engine to find this site, which I believe is British by the currency quoted. (I'm so tired of Google search engine giving all this ad related references right up front,...no meat to a search anymore)


https://www.makewoodgood.com/waterproofing-plywood-sealing-plywood-for-longer-life/



This site is promoting the use of clear penetrating epoxy sealer, and makes the case with test and photos. what I also found interesting is it specially states do NOT use PVA glues, and do NOT use water based products when attempting to seal up plywood.
 
Where I work, we use a wood sealing coating that resembles red paint. I can get the name of it tomorrow, so you can check into it. i do know it works very well.
 
beiland,

Take a look at this stuff:

https://www.cabotstain.com/products/product/Australian-Timber-Oil.html

I use it on the steps leading up to my porch as well as the timber floor in my Trailer (the type you pull behind a car). Both are exposed to the elements 24/7. I "painted" on the oil 2 years ago and the steps still look as good today as they did when first fitted, two years ago as well. The trailer was done 12 months ago and, again, still look as good today as when I bought it 12 months ago.

This is an actual "oil" so it is designed to penetrate the wood and seal it from the inside out, not just give the wood a "coating" like many stains. It is a bit more expensive than your run of the mill stuff but is worth every penny if you want to truly preserve your wood.
 
wombat457 said:
beiland,

Take a look at this stuff:

https://www.cabotstain.com/products/product/Australian-Timber-Oil.html

I use it on the steps leading up to my porch as well as the timber floor in my Trailer (the type you pull behind a car). Both are exposed to the elements 24/7. I "painted" on the oil 2 years ago and the steps still look as good today as they did when first fitted, two years ago as well. The trailer was done 12 months ago and, again, still look as good today as when I bought it 12 months ago.

This is an actual "oil" so it is designed to penetrate the wood and seal it from the inside out, not just give the wood a "coating" like many stains. It is a bit more expensive than your run of the mill stuff but is worth every penny if you want to truly preserve your wood.
Click to expand...
Looks expensive like a number of these competing products for outdoor deck treatments.

Their ad copy makes each one sound like the best thing since sliced bread, but I really don't need an 'outdoor capable' coating as my roadbed plywood will not be exposed to the harmful sun. I just don't want it to go thru all sorts of dimension changes with humidity and temp variations.

http://www.modelrailroadforums.com/forum/index.php?threads/track-or-subroadbed-expansion-contraction-problems.30195/
 
I’m with Tony, why not just use something like he suggested, or Thompson water sealer. These products should be plenty for your application.
 
Cabot's is good stuff, but Formby, or Minwax, Tung Oil will do the same protection. Personally I would go with Formby Tung Oil.
 
I would use T-111 siding and then waterproof it. Used it on my shed and I just painted it. It's not protected and out in the weather in all sorts of Montana weather.
 
Was thinking: Since you're going with very thick, (what ?, 7 layer), 3/4" ply, you may not need to do anything..As you know, ply is saturated with glue between each layer, making it 'hard as nails' along with each layer's grain running 90 or 180 Deg. to each other .. I lived in FLA and know what you're up against. During the daytime when you are not in the shed, will the AC be on ? Likely not. But as you say shed is insulated, I can't see much if any ply warpage..I'd worry more about the heat's affect on all the plastics above the ply. Finally: If the legs are wooden 2x3 or 2x4, they could buckle somewhat, under all the weight.. So I would likely seal them.....All the best, M
 
Dewaxed Shellac


maddragon
Greg, this topic always gets a lot of responses, most of them wrong. I've been a woodworker for longer than I've been a model railroader and that's a long time.
The best sealers are de-waxed shellac. or clear polyurethane. They're about equal but poly takes hours to dry and shellac takes minutes. Applying poly in a closed environment is harder on your lungs than shellac - unless you wear an appropriate mask.
For both reasons my first choice is the de-waxed shellac. It used to be that had to be mixed up from flakes, but for several years now Zinsser has marketed a 2 pound cut of dewaxed shellac as SealCoat. That's what I use.
Click to expand...

What is a 'two pound cut'?

I looked up the qt and gallon prices on this stuff and they want $20 per quart or $60 per gallon. Not exactly inexpensive.

Does it go a long way? Is it likely one only needs to seal the edges of the plywood, rather than the whole sheet front and back??

Brian
 
beiland said:
Their ad copy makes each one sound like the best thing since sliced bread
Click to expand...
beiland,

Your right their advertising does make it sound that way ... and from almost 40 years of experience with wood I can, without hesitation say - it is.

In your original post (#1) you said you wanted something to penetrate the wood. Paint, shellac, polyurethane does NOT penetrate the timber. Those products are designed to protect (albeit temporarily - a year or two) the timber by coating it. Polyurethane and shellacs will "peel" and yellow over time, even Marine Grade Shellac will deteriorate. An oil based product such as Cabot's, does penetrate the timber and does protect it from the inside out as the oil soaks into the timber. So DON'T be fooled or suckered into using a "cover up product" if what you really want is something to penetrate.

maddragon,

... this topic always gets a lot of responses, most of them wrong...
Click to expand...
As is proven by this comment:

... The best sealers are de-waxed shellac. or clear polyurethane.
Click to expand...
 
I went looking up Zinsser products the other day as a result of that 'shellac reference'. I found a number of them that claim to be sealers.

One in particular caught my attention,...
And then this mold & mildew-proof waterproofing paint, ...name would imply it will do the job,....& 15 year waterproofing guarantee
https://www.walmart.com/ip/Zinsser-270267-1-gal-Mold-and-Mildew-Paint-White/47647744


93112e47-4caf-4c5f-a8bc-a3493c75e73f_1.0f08f7732f217f1be22d3d1a8e4e8e2f.jpeg
Click to expand...
 
beiland said:
I'm building a layout in a shed external to my house in the hot, humid state of FL. While I have the shed under a carport and insulated well, it does seem to remain cool for the most part. But I do not intend to run the AC full time, so it will experience temp and humidity swings.

Many of my decks/shelfs will be good grade, multiply layered 3/4" plywood, which I intend to try and 'seal up' as much as possible against moisture absorption, and the subsequent warping that can occur.

My first thoughts turned to paints,...either oil or water based. But as I thought more about the situation I came to believe that the paints for the most part just coat the exterior of the plywood. They don't really penetrate the wood to seal it up.
Click to expand...

I agree with the earlier poster's (Mark) question as to why you should bother. Decent plywood is dimensionally stable due to its oriented panels glued together. Milled lumber is a different animal entirely. You would need a varnish or polyurethane if you wanted to come close to making it truly 'waterproof', and even that would be hit 'n miss.
 
Don't agree at all with the previous post!
Any plywood exposed to the elements will age and splinter (even the smooth sides) if not sealed.
Florida being subject to wild swings in humidity and moisture content - unless the "shed" is enclosed - over time the plywood will age.
An ounce of prevention is worth a pound of cure!
Especially if this plywood is used in the helix.
 
beiland,

Probably beating my head against a brick wall now but, polyurethane, shellac, paint are all sealers of one type or another and they do what the term "sealer" suggests, they seal whatever they go on. The problem with them is all they are is a product that sits "on top of" whatever it is put on, it is a "protective layer". They are designed to make things look all nice and shiny and pretty. However, If there are any gaps. missed spots, cracks or holes in the "sealer", if there are any thin spots in the "sealer" moisture will get in and that moisture will creep beneath the "sealer" and (in your case) the ply. If weather conditions where you are are that changeable so as to warp ply, the ply will warp no matter how much "sealer" you put on and that will lead to the sealer cracking. These "sealers" are little more than a covering.

You stated that you wanted something to penetrate the wood ... in that case the only thing that will do that is an "oil" which is absorbed by the wood. As such, it wont crack, shrink, discolor, deteriorate or leave any type of gap for moisture to get through. An oil is your only way of positively penetrating the wood and sealing it properly.

That being said, if conditions where you are are so severe that the issue of the ply warping is such a concern, then I honestly think you will have larger issues to contend with. Your best bet would be to close up your cross members (frame work) to 12" centers and securely screw the ply down. Doing so will reduce the possibility of it warping and might just stop it altogether especially as you are using 3/4 ply.
 
If the plywood is secured screwed ( especially the edges) to 1X4 framing, I wouldn't worry too much about any future problems. But, I would apply stain to all plywood surfaces and edges in the helix with a quality Deck and Fence stain like Behr's. (No commerical here) Like Chet noted, I have a cabin with T-111 plywood and its been on the cabin since 1970 surviving all swings in temperature and humdity in the woods of Wisconsin with no problems.

If the train shed is well sealed to the exterior elements, I would run a dehumidifier 24/7 to maintain decent humidity levels. High humidity, more than high temperatures would be harmful to the layout's scenery, wooden structures, metal parts and your personal comfort.

Would you consider setting the A/C unit at a high temperature and run 24/7 when you are not working in the train shed during the warmer periods in the summer? The A/C set to a high setting will cycle on and off as needed.

Greg
 
beiland said:
I went looking up Zinsser products the other day as a result of that 'shellac reference'. I found a number of them that claim to be sealers.

One in particular caught my attention,...
Click to expand...
I was at Walmart this morning and took a look at this product. The can was VERY heavy, and the liquid sounded very 'thick. I don't think a 'concrete/masonry' product will SOAK into wood very well, but rather just coat it.
 
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beiland said:
I was at Walmart this morning and took a look at this product. The can was VERY heavy, and the liquid sounded very 'thick. I don't think a 'concrete/masonary' product will soak into wood very well.
Click to expand...

It's time to make a decision on this! -- You are only confusing yourself at this point.
Either a penetrating oil product like Cabot, or Tung Oil -- OR a paint such as the Gripper (excellent product)

I made a baseball bat in shop back when I was a junior in HS.
I gave it 7-8 coats (applied with a rag) of Tung Oil. It took several days of drying between coats, but, except for a few scuffs and scratches, it still looks good after 60 years.
 
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