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Products: Exterior treatment for plywood

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    We're doing the ply window 'box' surround thing, with a 4-sided frame projecting out from our main structure. I paid a small fortune for marine ply that appears to be everything it promised - every passing tradesman on my site has praised it unprompted and it seems to be easy to cut, dimensionally stable etc. to boot.

    However, I'm aware that I need to treat the ply with something to make it properly 'external grade' as the marine spec just refers to the glues.

    Some online research has suggested something called Butinox but the best grade appears to have been discontinued (I guess due to the solvents involved). I'm also wary of using anything oil-based that my window installers will need to stick their membranes onto as I'm guessing the glues won't be warrantied for that kind of substrate.

    How have others using the ply approach finished their timber?

    n.b. The ply will be fully concealed by internal and external reveal boards and sill, so appearance is not my priority. I just want something that will preserve the wood from moisture (and in the event of a leak) and that the pro clima tapes will stick to well.

    Thanks in advance.
    • CommentAuthorborpin
    • CommentTimeFeb 2nd 2018
     
    I used sadolins on my ply. Exposed cut edge is the most vulnerable.
    • CommentAuthorMikC
    • CommentTimeFeb 2nd 2018
     
    I guess it's a bit late now, but if you want totally water proof ply then go for phenolic, it's what they use for trailer floors. Borax is water based but I'm not sure it will prevent rot..
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      CommentAuthordjh
    • CommentTimeFeb 2nd 2018
     
    I too used Sadolin and paid particular attention to make sure it soaked in to cut edges. If the ply is protected from the elements by overboarding then it shouldn't be at too much risk since the parts most at risk of wetting are also most exposed to the heat of the sun. I also put a membrane on top of the box and under the sill. BTW, we attached the boxes to the windows first and then inserted them into the wall wherever possible.

    If there's any difficulty sticking the tapes to the wood, then the first step is to use the RP primer on the wood and if that isn't enough, you can always see if some Orcon-F will stick to the primer.
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      CommentAuthorfostertom
    • CommentTimeFeb 3rd 2018
     
    Perhaps we shd use somthing other than wood-based, for these 'picture frames'? Supalux? CPB (cement particle board)?

    Apart from water leakage, from aged, damaged or faulty-installed flashings, these are in the poss danger zone from interstitial condensation by permeation and/or air-leak borne.
    • CommentAuthorMikC
    • CommentTimeFeb 3rd 2018 edited
     
    I like to look of MgO board. Some fireboards are MgO (magnesium oxide). I can't seem to find it thicker than 12mm though. I'll try and have a deeper look.

    The main benefits of ply is its so nice to work with. Not sure cement type boards are quite so easy to glue and screw for example.
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      CommentAuthordjh
    • CommentTimeFeb 3rd 2018
     
    Timber window frames are secured to timber studwork. Why are we worried that the connection is timber?
  8.  
    Supalux was (is?) very expensive and very brittle. Not the easiet thing to work with.
    • CommentAuthorborpin
    • CommentTimeFeb 6th 2018
     
    Posted By: djhTimber window frames are secured to timber studwork. Why are we worried that the connection is timber?
    Agreed.

    Posted By: Nick ParsonsSupalux was (is?) very expensive and very brittle. Not the easiest thing to work with.
    And not particularly strong in this context I'd suggest.
    • CommentAuthorborpin
    • CommentTimeFeb 6th 2018
     
    Posted By: fostertomPerhaps we shd use somthing other than wood-based, for these 'picture frames'?
    Hardly a picture frame. My windows are wood. Is that a problem too?
  11.  
    So I've just found a product called 'Clear Penetrating Epoxy Sealer' (I won't name the manufacturer but it's easily searchable). Appears to be highly regarded in the boat-building community for preserving plywood but it is also highly toxic.

    Not overly keen on the idea of covering my house in solvent based products, but it does appear to keep the ply fairly sound for much longer than a standard paint might. Since I'll be boarding over my window surrounds the lack of repainting certainly appeals.

    Anyone used epoxy or something similar in this way? Anything to watch out for, other than using a decent respirator when applying?
    • CommentAuthorgoodevans
    • CommentTimeFeb 15th 2018
     
    I think applying somthing like that to the cut edges of the plywood might be all the treatement you would need if going for WPB or Marine plywood.
    • CommentAuthorringi
    • CommentTimeFeb 19th 2018
     
    Remember it will stop the ply from "breathing", therefore increasing the risk of condensation from behind.
    • CommentAuthorborpin
    • CommentTimeFeb 19th 2018
     
    Posted By: ringiRemember it will stop the ply from "breathing", therefore increasing the risk of condensation from behind.
    As the ply is perpendicular to the structure this is hardly an issue.
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      CommentAuthordjh
    • CommentTimeFeb 19th 2018
     
    Posted By: borpin
    Posted By: ringiRemember it will stop the ply from "breathing", therefore increasing the risk of condensation from behind.
    As the ply is perpendicular to the structure this is hardly an issue.

    It still has an inside and an outside.
    • CommentAuthorgoodevans
    • CommentTimeFeb 20th 2018
     
    I did recommend only applying to the cut edges - I think the rest of the surface is best left alone to do the job as described (wbp, marine whatever)

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