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Old 09-14-2020, 07:54 AM   #1
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Advice for protecting exposed wood on underside of trailer

Hi guys. I could use some ideas for how to prevent the wood underside of my trailer from rotting. I have a custom built 4x3 covered trailer trailer that was essentially built using the NorthernTool kite found here.



An untreated wood plywood deck was created for the floor and a frame built above to enclose and it now looks like this.



So the issue is that a treated piece of wood wasn't used for the floor and the way it was designed, the top cannot be removed to have the floor replaced. The underside of the trailer doesn't feel wet, but it looks somewhat discolored and I'm concerned for the longevity of it.

I thought about using several coatings on the underside of Flex Seal (which I already have) and a caulk sealant such as Weathermaster on the bottom to seal betwen the metal frame and the wood.


However, I'm uncertain if I will potentially be sealing moisture in - or worse if it does get on top, will it be prevented from drying out because of the seal. Would a wood deck sealant be better? Am I overthinking this??
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Old 09-14-2020, 10:50 AM   #2
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I don't know that sealing anything after installation is going buy you much. The rot will happen where the plywood is joined to the frame. It will wick in and not readily dry like the open spaces. It's probably only the open spaces you'll be able to treat.

Do you know what the exposure rating for the plywood is? How often are you towing in wet conditions?
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Old 09-14-2020, 11:34 AM   #3
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What do you mean by exposure rate? It sits outside exposed normally, although I will be planning to cover it with a tarp now. I typically only use it once a year when I drive it 850 miles to the east coast and back. I may encounter rain, but never know.

You are correct in that I could only treat the area that is not directly covered by the metal frame. However, I was planning to caulk seal where the metal touches the wood. That wouldn't help prevent moisture from wicking? I'm guessing you're right though and moisture will tend to find its way in regardless and then be locked in there. Maybe sealing it would make it worse. Was just hoping there was something preventative I could do...

Edit: I just looked and it appears that there is house wrap between the metal and the wood. But it only is where the wood and metal are touching.
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Old 09-14-2020, 03:20 PM   #4
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The bigger problem will be if you tow it in wet weather. I would have used marine plywood but it is a little late for that advice now. You may want to coat it with a good undercoating product.
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Old 09-15-2020, 05:38 AM   #5
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Coraplast and spray undercoat.
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Old 09-15-2020, 07:50 AM   #6
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Quote:
Originally Posted by jadon View Post
What do you mean by exposure rate?
Plywood has a rating system that is supposed to give you an idea of how long it can be exposed before the adhesives break down and it delaminates. If it's not even exterior rated, it will break down quicker if it gets wet. It should be stamped somewhere. Exterior or Exposure 1 will probably be fine. Exposure 2 or Interior, you'll want to try and supplement the waterproofing.
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Old 09-15-2020, 08:12 AM   #7
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Plywood has a rating system that is supposed to give you an idea of how long it can be exposed before the adhesives break down and it delaminates. If it's not even exterior rated, it will break down quicker if it gets wet. It should be stamped somewhere. Exterior or Exposure 1 will probably be fine. Exposure 2 or Interior, you'll want to try and supplement the waterproofing.
I wasn't able to locate this stamp but it's small enough that it may have been cut off.

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Originally Posted by Toby Dog View Post
Coraplast and spray undercoat.
Hmm. This is an interesting idea. I could use this to overlap the L-angle of the frame on the underside. I could then use wood glue or gorilla glue for adhesive. Then caulk all around where it butts up to the metal. My only concern is that if moisture does get in from somewhere, then it will just sit between the plastic and the wood - causing excellerated rot.
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Old 09-15-2020, 08:16 AM   #8
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It is built, so any thing now will be a patch. See if you can get someone to apply a spray on coating of tar or a bed liner to the whole underside.

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Old 09-15-2020, 08:27 AM   #9
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Get a few cans of spray undercoating, the asphalt based kind and not the rubberized stuff. Hit the bottom at a car wash to get it good and clean, give it a couple of days to dry thoroughly.
Coat the bejeezuz out of the bottom with the undercoating.

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Old 09-17-2020, 07:32 AM   #10
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Thanks for the all the advice guys. After additional research I have read several accounts where waterproof sealers such as Flex Seal and spray on under coatings like the asphalt types usually results in trapping moisture and water between the seal and the wood which leads to accelerated rot. I believe it may be best for my scenario to instead use something like linseed oil which can still protect against the moisture and allow the wood to breathe at the same time.
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Old 09-17-2020, 07:49 AM   #11
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Any coating you put over the exposed plywood will just trap moisture inside and accelerate the rotting process. Eventually you will need to replace the plywood floor. My advice would be to just use the trailer as it is until the rot gets bad, this might be 5-10 years from now.

Hopefully the chip board plywood was not used for the floor which would only give you 3-5 years of use.
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Old 09-17-2020, 08:13 AM   #12
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Ok, take used motor oil and spray on it before the undercoating.
Thats why I said an asphaltic undercoating instead of the rubberized garbage.
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Old 09-19-2020, 07:55 PM   #13
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Thompsons water seal, once the sealer is dry, calk around frame points where you can't get to the plywood. Then paint it with a good quality exterior paint.
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Old 09-19-2020, 08:35 PM   #14
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Unless you can coat all sides of the wood coating the bottom will just hold any water that gets in from the top.
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