23402Re: [Michalak] MDO
- Sep 26, 2011Good post nels and accurate!
From: prairiedog2332 <nelsarv@...>
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.
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