On Fri, 14 Oct 2005 01:28:20 -0700, ahoy wrote:

ahoy, I epoxied some one inch strips of wood in slats to make up a
little decorative rope bin which was intended to sit on deck but it
warped and popped apart in about 3 days in a damp shop. Does
everything have to be notched or biscuited or doweled and pinned or
screwed? I thought epoxy was an invincible adhesive. I used left over
strips of purple hart and mahogany and west system 105/205 in a dry
room if that makes any difference. thanks as always.
There is epoxy and then there is epoxy. In other words there are various
formulations of various quality for various purposes. Oak in particular has
problems (the tanin in it). Unless the wood was epoxy coated the force of the
wood warping exceeded the strength of the joint.

Epoxy is an adhesive, it is not a miracle substance of unlimited strength.

Squeeze too much out and the joint is weak. Have too thick of a layer and
again the joint is weak. Some formulation are known to suffer from excessive
creep, but those are intended for finishing not wooden joints. All epoxies
soften at warm temperatures, the point depending on the formulation. But a dark
hull on a sunny day can easily exceed that point. It is also waterpermiable.
It doesn't let in much but it is not like a metal barrier, some moisture will go
through epoxy, the amount and speed depending on the formula and the thickness.

For very highly stressed wood to wood joints I would either use resorcinol or
epoxy with the joint REINFORCED by fiberglass cloth. De lamination with epoxy
is not uncommon when the wood is not completely and properly epoxy coated to
control moisture.

Epoxy resin is much stronger and less permeable than polyester resin, but again
it is not a miracle substance. It has limitations just like any other material.

JJ




James Johnson
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The problem wasn't the epoxy. The glued connection wasn't strong enough to
resist the warping ...which can be quite strong I might add. You can either
back the joint up with hardware or encapsulate the wood in epoxy so that it
doesn't absorb moisture. With proper preparation of the wood(s) and proper
mixing of the epoxy, the wood generally fails before the epoxy. In other
words, when the joint gives way, you'll find all the epoxy and some torn
away wood on one side and nothing but wood on the other side. The only time
that I had an epoxy joint fail (other than doing tests), it failed right in
the epoxy itself but it was a joint that I remembered may not have had the
right resin/hardener ratio in it. My cheap bottle top pumps failed me when
one went 'pop' and shot some air into the cup instead of epoxy. I tried to
guess a corrective amount to squirt in and just took a risk. The epoxy was
whitish and crystallized, brittle. NOT normal. The lesson there was to
avoid guessing and to make sure the measured ratio is correct, then mix
well. But in your case, it sounds like if the wood is not going to be
encapsulated with epoxy (coat 3 times), then a mechanical back-up is in
order. If you're not going to encapsulate, and especially if you'll put
your project in a damp environment, then yes ..treat epoxy like any other
glue and dowel it, or back it up with screws, or whatever. I've seen
warping wood pull screws out ...nothing is invincible, even epoxy.

Brian D



"James Johnson" wrote in message
...
On Fri, 14 Oct 2005 01:28:20 -0700, ahoy wrote:

ahoy, I epoxied some one inch strips of wood in slats to make up a
little decorative rope bin which was intended to sit on deck but it
warped and popped apart in about 3 days in a damp shop. Does
everything have to be notched or biscuited or doweled and pinned or
screwed? I thought epoxy was an invincible adhesive. I used left over
strips of purple hart and mahogany and west system 105/205 in a dry
room if that makes any difference. thanks as always.
There is epoxy and then there is epoxy. In other words there are various
formulations of various quality for various purposes. Oak in particular
has
problems (the tanin in it). Unless the wood was epoxy coated the force
of the
wood warping exceeded the strength of the joint.

Epoxy is an adhesive, it is not a miracle substance of unlimited strength.

Squeeze too much out and the joint is weak. Have too thick of a layer and
again the joint is weak. Some formulation are known to suffer from
excessive
creep, but those are intended for finishing not wooden joints. All
epoxies
soften at warm temperatures, the point depending on the formulation. But
a dark
hull on a sunny day can easily exceed that point. It is also
waterpermiable.
It doesn't let in much but it is not like a metal barrier, some moisture
will go
through epoxy, the amount and speed depending on the formula and the
thickness.

For very highly stressed wood to wood joints I would either use resorcinol
or
epoxy with the joint REINFORCED by fiberglass cloth. De lamination with
epoxy
is not uncommon when the wood is not completely and properly epoxy coated
to
control moisture.

Epoxy resin is much stronger and less permeable than polyester resin, but
again
it is not a miracle substance. It has limitations just like any other
material.

JJ




James Johnson
remove the "dot" from after sail in email address to reply