exterior plywood question

Discussion in 'Materials' started by minno, Feb 16, 2015.

  1. minno
    Joined: Aug 2014
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    minno Junior Member

    Hi All

    just wondering if there's any reason why I can't use a lower quality exterior plywood and fill it with thickened epoxy if I'm going to paint the boat anyway?

    As I understand it the core is the same in all exterior plywood and the only real difference in the outside plys is cosmetic.

    It would certainly reduce costs.

    take care

    minno
     
  2. TANSL
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    TANSL Senior Member

    I am not a builder but would advise you, on ships, use the best quality possible. The best, in general, it is cheaper in the long run. Even cosmetics, for boats, must be the best due to very aggressive environment in which you work.
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    TANSL has given you the right advice, don't spoil the ship for a penny's worth of tar.
     
  4. SamSam
    Joined: Feb 2005
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    SamSam Senior Member

    Scroll down to the bottom of the page to 'Similar Threads', it's a common question that has been discussed a lot.
     
  5. minno
    Joined: Aug 2014
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    minno Junior Member

    Thanks guys :)

    If I had the skills and experience to build a ship I'd know the answer to the question :)

    I've got 4mm okoume and AB grade ext. fir for the duo I'm going to build, but I've never built a boat before and am considering building a small 2 or 3 sheet rowboat out of cheap materials for practice, I'll probably give it away when I'm done my duo.

    so is there any reason not to do it other than because it won't be quite as good?

    thanks again

    minno
     
  6. minno
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    minno Junior Member

    thanks, missed that somehow

    take care

    minno
     
  7. JR-Shine
    Joined: May 2004
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    JR-Shine SHINE

    1) voids or lack thereof.

    2) Numbers of veneers and their quality

    Voids with in a panel with fewer veneers will make a panel that is not nearly as strong as one with no voids and more veneers.

    Example: 3/4" exterior fir will typically have 5 or 7 veneers. It will have some voids inside. 18mm Meranti we have is 13 solid veneers

    If you using the fir as a core and piling a bunch of glass over it, then it probably will be as good as the marine ply.
     
  8. minno
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    minno Junior Member

    Thanks JR :)

    I got to thinking that with the price of epoxy it might actually cost more to fill cheap plywood than buy a better grade of exterior, doh :)

    sorry if I seem cheap, I'm just following the instructions and looking for a piece of scrap to try it on first before I build me a boat.
    If this practice boat turns out to be a corkscrew I'll paint it up nice, plant flowers in it, and use it for a lawn ornament.:D

    take care

    minno
     
  9. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    If the plywood has an "APA" stamp or label it needs to read "Exterior". There are two exterior grades and they look similar, including the label. The one you don't want is marked "Exposure 1" or "Exterior Exposure 1". This lesser grade of Exposure 1 is intended to be used as sheathing that can tolerate an occasional rain shower, before being covered with house wrap or some sort of siding (vinyl, stucco, etc.). It will rot quickly if it sees much moisture and it's internal defect count is pretty high.

    [​IMG]

    For a jon boat or other, simple low cost build, regular "Exterior" will do, though it's not as strong, nor will last as long as a real marine grade sheet. If you're lucky enough to get an APA marine grade, it'll be marked "AA Marine" and usually has a "PS 1-09" mark on the label too.

    [​IMG]

    This is the one you want, of course in the thickness you need. The top one is the one to avoid.
     
  10. JR-Shine
    Joined: May 2004
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    JR-Shine SHINE

    that is a valid point if your only concern is the fairing and checking/print through, but that does not help the inside.

    I would add a little glass (even if its just a 4 oz woven) to the outside of any fir plywood just to be sure the grain did not print through over time.

    I agree, and if you can find 5 ply underlayment plywood, I would not hesitate to use it on a dingy or small planing boat - just have to be certain its the 5 ply with exterior grade glue
     
  11. Ilan Voyager
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    Ilan Voyager Senior Member

    Building a boat, except a very cheap kleenex prame, from cheap exterior plywood is a pain, specially fir which is almost impossible to sand smooth and which checks badly even with fiberglass and epoxy. Fir marine plywood works for small working boats, with just paint. The treated one (green) will warp badly but it's excellent for some applications. PAR gave you the tip.
    After spending hours of work and too much money you end with a inferior product.
    If your okoume is a 3 ply with thin exterior plies and a thick core forget it. If the glue lines are clear and not dark it won't stand the water. If it has not stamp on the sheet with the specs, it's for closets. Not even for flowers.
    May we know the brand of the okoume plywood and if it has red or blue stamps like "CTBX" on it?
    There are plenty of threads and a few years ago I gave the methodology for testing plywood.
     

  12. minno
    Joined: Aug 2014
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    minno Junior Member

    Hi All,

    thanks for all the great info :)

    sorry, I should have posted back, I ended up up buying a couple sheets of 1/4" fir exterior, clear/sanded one side, filled/sanded (fir plugs not wood filler) on the other, it's 3 ply and the salesman said it had very few voids.

    there's no stamp but I figure that's because it's meant to be finished bright on both sides. I hope it will be adequate for the small duct tape and glue canoe i'm building http://www.bateau2.com/free/ccanoeUS.PDF to learn the basic skills.

    getting plywood here is a real challenge, I don't drive so I'm pretty much limited to what the local stores carry or are willing to bring in.

    I've already got a couple sheets of okuome BS1088 for the boat I'm practicing for and I'll use the best lumber I can find for the rest.

    Thanks again

    Take care

    minno
     
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