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28017Re: [Michalak] Re: Plywood checking on Brucesboat

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  • Joseph Stromski
    Jun 24, 2013
      The MDO I used had non-primed facing on 2 sides. There's quite a few different varieties of it out there, however. Primed, non-primed, etc.  I handled it no differently than regular plywood. The boat is garaged. 


      From: Brian Nimmo <owwin@...>
      To: "Michalak@yahoogroups.com" <michalak@yahoogroups.com>
      Sent: Monday, June 24, 2013 11:23 AM
      Subject: RE: [Michalak] Re: Plywood checking on Brucesboat

      Hi Joe,

      Does the MDO have the nice facing on both sides?  Does the facing adhere to epoxy like normal plywood?  How do you store your AF4 during long periods of non-use?



      To: Michalak@yahoogroups.com
      From: j.stromski@...
      Date: Mon, 24 Jun 2013 09:12:56 -0700
      Subject: Re: [Michalak] Re: Plywood checking on Brucesboat


            I agree with everything below, except the "pricy stuff" comment. My AF4 used 1/2" MDO for the bottom, and 1/4" baltic birch premium underlayment for the side panels. Both obtained cheaply from my local big box store (Menards). The MDO hasn't/won't check due the the facing on it, and the birch shows no indication of checking with only an epoxy spit coat, no glass, after nearly 4 years.




      From: "Crandall, Chris S." <crandall@...>

      To: "Michalak@yahoogroups.com" <Michalak@yahoogroups.com>

      Sent: Monday, June 24, 2013 9:49 AM

      Subject: [Michalak] Re: Plywood checking on Brucesboat


      In short, epoxy painted over plywood does NOT stop checking, although it slows it down some. To prevent checking, there are two choices:

      1) Use a plywood that is not prone to checking. This is the pricy stuff, made overseas, and usually involves cutting down some of the rainforest.

      2) Use fiberglass cloth with that epoxy.

      Checking comes from moisture cycling. The earlywood and latewood in douglas fir (in particular) expand and contract and notably different rates. This, in plywood veneers, leads to tearing of the fibers (checking) with repeated cycling. Plywood kept at constant moisture doesn't check--but what boat doesn't moisture cycle? (Boats kept in museums, perhaps.) Since tropical rainforests have more constant climate/seasons, there is not the pronounced earlywood/latewood structure--that's the rings you see in the wood [try counting rings in teak or iroko]--and so the checking is less of a problem.

      So the painting on of epoxy slows down, but as you've learned, does not prevent checking. Since you're not going to replace the plywood on your boat, you have a few options:

      1) Sand, sand, sand to bare wood, apply a lightweight fiberglass cloth (4 oz is fine, 2oz might be OK), set in epoxy.

      2) Sand some, and patch. This is the easiest, and it isn't bad. Sand it down as much as you can stand, and then patch with epoxy and filler. I have come to like WEST System Microlight filler. It sands very, very easily. I will never go back to phenolic microballoons. (It's also a pleasing color, which the maroon balloons were not.)

      3) Paint another coat over it, and recognize that checking is only a problem for you if they trap moisture. If you can keep the boat dry, your problems are primarily cosmetic.

      And remember to enjoy your boat as it is, and to plan another build.


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