Advice For Treating Plywood For External Use

Discussion in 'Carpenters' Talk' started by Bazzy, Oct 15, 2021.

  1. Bazzy

    Bazzy New Member

    Hi All,

    I have a number of long 18mm Plywood boards about 2m+ long & 600mm wide that have either been treated, stained or oiled - not sure which. They were boards placed in my loft which have been replaced.

    Not convinced by the expensive plastic Keter types of garden storage boxes, I wish to use the Plywood instead to make them but would guess it would need to be weather & water-proofed - can anyone advise me on what would be the best way to do this & ensure a lasting result please?

    Many Thanks
     
  2. Jord86

    Jord86 Screwfix Select

    Building your box then cladding it would be your best bet for longevity.
     
    Bazzy and rogerk101 like this.
  3. AnotherTopJob

    AnotherTopJob Screwfix Select

    If the exterior surface of plywood is well protected it should be quite durable, however eventually damp can cause de-lamination, even in exterior grade. Protecting the end grain will help.
     
    Bazzy likes this.
  4. Bazzy

    Bazzy New Member

    Hi,

    Thanks for the input - can you kindly let me know of what type of cladding you are referring to please?
     
  5. Bazzy

    Bazzy New Member

    Hi,

    I am not sure what grade of Plywood it is but considering that it was used to board up the loft, I doubt that it would be be exterior or marine grade. It is treated with something as it is somewhat tacky to the touch, much darker in colour than normal & I can see the stain/oil streaks on the edges.

    Apart from the cladding that was mentioned, is there some special kind of paint/coating that can be applied to the plywood to weather-proof it against the elements?

    Thanks!
     
  6. AnotherTopJob

    AnotherTopJob Screwfix Select

    It really needs a good exterior varnish or paint, I don't think a fence-type treatment will prevent water ingress.
     
    Bazzy likes this.
  7. Bazzy

    Bazzy New Member

    Hi,

    Can you recommend any particular type/brand that would be suitable? I know Ronseal apparently do specialised coatings but not sure if any for Plywood for external use. I saw a You Tube video for something called Liquid Rubber designed for Plywood but it was a USA product that I do not think is available here - anything similar?

    Many Thanks!
     
  8. AnotherTopJob

    AnotherTopJob Screwfix Select

    Not sure on brands but a polyurethane varnish should do the job if you're happy with the existing colour, or an oil based primer/gloss of your choice (Dulux weathershield is good).
    Your existing stain needs to be sanded or removed to ensure good adhesion.
     
    Bazzy likes this.
  9. Bazzy

    Bazzy New Member

    Hi,

    So I am happy with the existing colour then all I have to do is apply this Polyurethane Varnish (without sanding?) - can it be sprayed on with a garden sprayer or just applied by paint brush? How many coats to ensure full weather-proofing & last many years?

    If she wants a different colour, then sand boards all round & then apply the Dulux Weathershield? How many coats please?

    Thanks Kindly!
     
  10. AnotherTopJob

    AnotherTopJob Screwfix Select

    If your existing wood is sticky with a previous treatment, it's not a good basis for any paint. Sanding or white spirit should remove it.
    2 coats with a brush should be fine, but follow the instructions on the tin. Spraying would probably be difficult unless you thinned it right down, which is not ideal.
    Make sure you seal the end grain.
    Painting over with paint later should be fine but you'd need to let it cure for a good few weeks before sanding again.
     
  11. Mr Rusty

    Mr Rusty Screwfix Select

    IMO nothing protects ordinary plywood outside when the end grain is exposed. However, I have used 2-pack west systems epoxy coating to finish a plywood sink for a darkroom, which lasted several years with no degradation until we moved (but it isn't cheap). I will be building another in the not too distant.
     

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