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Thread: A Cautionary Tale In Regards To Plywood

  1. #1
    DIYFK member Meep's Avatar


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    A Cautionary Tale In Regards To Plywood

    I just want to put this out there, I believe I have said it before but it's worth saying again... If you do build a plywood tank, seriously and I means seriously consider using a true 'cabinet' grade high ply count plywood vs the big box store crap plywood...

    I picked up a bunch of BC grade plywood to build parts of my beehives and other stuff from a local big box store, and after cutting it into multiple smaller pieces I was quite shocked to see all the voids in the plys, these voids will most certainly compromise the integrity of a plywood tank, especially if the void is under the first ply facing the water...

    On one board I cut I happened to cut about dead center into about a 2" round void on the ply right behind the 'good' B grade sanded side, making a 2" long section along the edge of the cut piece wher the 'finished ply was literally flopping around the edge with nothign behind it, I ended filling the void with my own new layer of ply and wood glue to return integrity, but if that void had been in the middle of a board used in a tank build, you can bet it would have collapsed into a visible give or take 2" round divet when filled with water and probably caused the failure of the epoxy sealing on top of it due to flexing... The other multiple smaller voids in the plys I saw when cutting would certainly not help either...

    I know the cost of a good sheet of plywood is double or more of what you will pay at a big box store, but it's really cheap insurance and worth the extra money for a quality build...

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    DIYFK member FLDave's Avatar


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    As I used to tell my customers......You'd be amazed at just how expensive "cheap" can be......Click here to enlarge

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    DIYFK member Meep's Avatar


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    I was out tidying up the garage and came across this drop that shows what I found repeatably in the sheet I cut up...

    You can see the one void is entirely through this give or take nearly 2" wide drop, and it continued into the piece it was cut from, you can also see another void (and crappy ply) near the edge of the same board, as you can see from the top laminate picture this sheet appears to be of decent quality, but it's not... Just for reference, this is a $35 sheet of plywood from a big box store...

    void_1.jpg
    void_2.jpg
    void_3.jpg

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    DIYFK member HillbillyHomer's Avatar


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    WOW!

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    DIYFK member sloanrd's Avatar


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    Yeah, Wow is right. What are your thoughts on using marine grade plywood? Is it same as regular old plywood?

  6. #6
    DIYFK member Meep's Avatar


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    Like regular plywood there are many grades of marine plywood, basically, marine grade just means that a waterproof glue was used and it's supposed to not have voids, it could still be crappy wood...

    Marine-grade plywood is a specially designed panel made entirely of Douglas-fir or Western Larch. The grade of all plies of veneer is B or better, which means it may have knots, but no knotholes. The panels are sanded on both faces, and are also available with Medium Density Overlay (MDO) or High Density Overlay (HDO) faces. The maximum core-gap size permitted is 1/8 inch. Its exposure durability rating is EXTERIOR and the glue used is a fully waterproof structural adhesive. It is considered a “premium” panel grade for use in situations where these characteristics are required, i.e., for boat hulls and other marine applications where bending is involved.
    Personally, I would stick to high layer count cabinet grade Baltic birch plywood, it's a dream to work with, and even though I too cringe at the price, overall when building a tank it's a small upcharge for what you get...

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