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23402Re: [Michalak] MDO

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  • Andres Espino
    Sep 26 4:36 PM
      Good post nels and accurate!

      From: prairiedog2332 <nelsarv@...>
      To: Michalak@yahoogroups.com
      Sent: Monday, September 26, 2011 3:55 PM
      Subject: [Michalak] MDO


      There are several misconceptions regarding MDO.

      I think the first has to do with the "paper" overlay. Some group
      postings have recommended this has to sanded off for best adhesion when
      glueing MDO to framing. The overlay is actually cellulose-based - same
      as the wood - and is bonded to the plywood using high heat and
      resin-based glues same a the glues and heat used to make the plywood in
      the first place. From what I understand. the bond is stronger than the
      the plywood face.

      The overlays main purpose is to prevent checking of the underlying fir
      face. So in a way it is equivalent to adding a thin layer of fiberglas
      - the usual way to prevent checking in fir plywood. It is very weather
      and water resistant - more so than exterior grade fir plywood would be
      otherwise, and I think even more than marine grade fir. MDO is claimed
      to be less than 6% water absorbable.

      If it is futher protected by epoxy impregnation and a water resistant
      marine grade paint then the resistance goes up even further and if you
      also glass the areas below the waterline with epoxy you get about as
      close to 100% as is possible several builders who use MDO suggest.

      The drawbacks are: Fir plywood is heavier compared to the more expensive
      exotic marine grade plywoods and it is not available in usual
      thicknesses since the overlay adds its own thickeness. So no 1/4" is
      available for example. 1/4" becomes 3/8" and is heavier to boot than
      1/4" exotic marine grades in the first place.

      MDO is also not entirely free from "print through" of the underlying
      grain of the fir underlying plys. So one has to accept that possibilty.
      Otherwise it works every bit as well as the exotics at a lot lower cost.
      Any print-through is a mere ghost in my experience unless one looks very

      Relating all that to Michalak designs I woild consider a 3/8" MDO bottom
      on a hull designed for 1/4" plywood and consider Sureply for the
      topsides - which won't really get all that wet even if not glassed and
      not likely subject to checking if not glassed. Otherwise go with 3/8"
      MDO and accept the weight if that can be tolerated.

      Money will not be wasted on a good quality paint in the final analysis
      though, regardless of the choice of plywood.


      [Non-text portions of this message have been removed]
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