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Old 01-19-2012, 06:02 PM   #1
Eric Johansen
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How to waterproof new sub-floor?

What product do you recommend to waterproof a new plywood sub-floor?

Thanks, Eric

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Old 01-19-2012, 06:17 PM   #2
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Not that any water-proofing is considered long term, but I used marine epoxy on my marine grade ply.

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Old 01-19-2012, 07:10 PM   #3
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I used a product from Sanitred. It is a liquid rubber coating application. It has may different applications. i especially like the fact that it is used to make aquariums. The wood will be completely waterproof once the product is applied. Not water resisitent but water proof. It is very pricey though. I didn't mind the cost as I never want to need to repair my follr again.
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Old 01-19-2012, 07:34 PM   #4
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I used West systems epoxy around the edges of my MGP sub floor then used spar varnish on the rest
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Old 01-19-2012, 09:32 PM   #5
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Hardware store epoxies may not be waterproof. Look for boatbuilder/marine epoxies. System Three and MAS are good; West System probably is a more expensive option.

When I re-did areas of floor on my Argosy, I used System Three on the plywood edges and six inches on top of the floor perimeter. And then I treated the rest of the floor-top (eg, under the water tank) with RotDoctor - a high-solvent penetrating epoxy (ventilate nothin' -- gotta do this with a lot of outside air circulation!). Don't count on System Three, MAS or West to penetrate. Encapsulating wood really is a vain pursuit -- moisture will still penetrate somewhere and then nothing can evaporate (resulting in accelerated rot). I left the underside of my floor untreated.

Marine epoxies can be dealt with in enclosed spaces. Nothing more than a slight ammonia smell. But be very good on glove wearing and wash any skin contact with un-set resins. Use respiratory protection if you power finish or power sand any epoxy treated area.

Time spent looking at window gaskets, roof penetrations & skin lap seams will be more important than going overboard on sealing the floor. Gotta take care of the 'they all leak' problem.

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Old 01-19-2012, 10:38 PM   #6
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Lightbulb Thinking ahead...

What kind of floorcovering will you be putting down? Some "glues" will not stick to the :waterproof" surfaces. If you are just laying carpet or something that isn't glue-down, no problem. If you plan on installing linoleum or tiles, you may want to think twice before applying a waterproofing material wall-to-wall.

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Old 01-20-2012, 06:59 AM   #7
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Polyurathane wood coating works well and it is relatively cheap. It is also waterproof assuming there are no holes in the coating or gaps. Edges are especially important that they are sealed. You want to seal any penetrations through the wood as well. If the floor stays wet long enough eventually water will penetrate through some pinhole or crack in the coating etc. Many so called fiberglass boats are just fiberglass coated wood. The wood rots in 10 or so years and you have a fiberglass egg shell where your boat use to be. The advice of coating the top and the edges and leaving the bottom bare is probably not a bad idea if there is nothing like pink fiberglass that is holding moisture against it. The bottom edges should be sealed for 2 or 3 inches from the edge though.

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Old 01-20-2012, 07:13 AM   #8
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Another thought:
Look at Tnemic coatings. ( Tnemec Industrial Coatings and Architectural Coatings ) I have used this on concrete to waterproof the interior of an indoor water feature. I believe they have a product that will work on wood as well. It will be pricey but bulletproof.
A different option would be a peel-and-stick membrane variety. I have used a product called Proflex. Is a waterproofing membrane as well as a sound reducing mat. The primary application is for tile underlayment, however I have used it under wood floors in multiple applications with great success. PROFLEX? Products - Crack Suppression - Waterproofing - Sound Control - Membranes Underlayments

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Old 01-20-2012, 10:15 AM   #9
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I replaced the back sheet and then realized the whole floor had to go. I know, poor prior planning but I kept hoping to find where one gawd awful smell was coming from but even with bath, tanks, and all plumbing out it persisted.

I wasn't happy but I learned a couple of things that I'll share..

I used marine-grade plywood and put 4+ coats of vinyl-ester resin on before installing it. The first two coats were thinned and the wood was saturated to the point of rejection, I just kept washing it on until the soaking in slowed way down. Two more un-thinned coats and I though I was set.

Vinyl Ester resin is superior in every property as compared to all polyester resins. This product is used in high performance applications in which high-strength and maximum heat tolerance is necessary. This resin is extremely chemical resistant when cured. Similar in strength to epoxies with much increased stiffness.

Vinyl-ester is brittle - although it armored the surface the underlying wood yields and the VE cracks like rock-candy. When the wood grain shifted from moisture content the VE failed. See picture, the MGP on top of the pulled 40-year-old flooring.

The 8-ply the private lumberyard sold me was more furniture grade than construction grade. If you go MGP research the grades and symbol stamps and review the offerings available before you commit. The $85 sheet I got had a soft exterior ply and even softer internal plies - I had floor screws pull halfway through with little effort.

The new floor I just put in is a hardwood construction grade plywood (MDO) and I followed directions meant for outdoor advertising billboards: Fill any flaws w/ caulk and two (three in my case) coats water-based primer, then two top coats of outdoor porch paint. If you want to rubber-neck along with my slow-motion train wreck restoration look for a post "Hey y'all, watch this..."

If I were doing a back-sheet restoration again I'd seriously be tempted to use an epoxy resin that would allow thick build up, kinda like a surf board but that is just me.
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Old 04-08-2018, 03:09 PM   #10
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subfloor and bilge paint

i am replacing the rear-end subfloor. i am planning on using marine plywood. my next thought was to paint it with bilge paint. any thoughts? any problems with reactivity or corrosion due to aluminum and that type of paint? i see many use an epoxy or spar urethane.... thanks
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Old 04-08-2018, 03:16 PM   #11
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Originally Posted by pilar View Post
i am replacing the rear-end subfloor. i am planning on using marine plywood. my next thought was to paint it with bilge paint. any thoughts? any problems with reactivity or corrosion due to aluminum and that type of paint? i see many use an epoxy or spar urethane.... thanks
Welcome to the forum.

Stick with a good epoxy on the marine plywood. It will last longer than we will
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Old 04-08-2018, 03:21 PM   #12
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Or you can just use Coosa board, not have to treat it with an expensive epoxy and not care about leaks into your subfloor. It won't rot, mildew, grow mold; 40-45% lighter than plywood, stronger than plywood and just as easy to machine.

Pricey, maybe, so is marine plywood and West epoxy; peace of mind that nothing will rot your sub-floor.....priceless.

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