"JustWait" wrote in message
...
In article 434f169d-b755-48a0-91f5-
,
says...
My three yr old Tolman 20' Standard Skiff has some serious epoxy
problems that cause me to doubt that this building method can work for
my N. FL conditions.
I built her with System 3 resin using the slow hardener due to the
heat here. I used the metering pumps o get correct ratios and
followed the builders directions.
All went well for the first two years but then got a separation of
epoxied wood at the transom. This had been glued by first painting
the wood with epoxy followed by a thin layer of epoxy/fiber paste and
serious clamping. I pried the two layers apart last year and re-
filled with epoxy thinking the problem solved (NOT).
Yesterday, I found the biax separating from the stringers, a long
piece that had been clamped to the gunwales and glued separating and
other places where the epoxy simply had not adhered to the wood. I
will be able to use her for the next couple weeks (scallop season) but
this will require some major surgery.
Under N. FL humidity and rain, it is nearly impossible to keep her dry
and I suspect that swelling of the wood due to absorption of water is
the big problem.
I plan to remove all the rest of the expensive paint i put over the
original epoxy primer because it simply did not adhere anyway. Then
will grind out the bad areas dry everything and try to re-glass the
bad areas after using a heat gun to dry them.
I would hesitate to use epoxy/glass over wood for another project here
in N. FL conditions.
Hey Froggie, Scotty here from that other group
I think I remember back
when you were doing this, did you encapsulate the wood in Epoxy? I have
always been against that, moisture gets in, but can't get out. I suggest
regular paint as a sealer, just for the record...
FWIW, Paul Oman nailed the major issues that look like a possible failure
route here. First, how long did you wait between painting on the original
coat of epoxy and applying the paste? If it was too long, amine blush is a
possible culprit. Depending on the mix, 12 - 15 hours can be too long. 24
hours is more likely the max. After that, you have to remove the amine blush
with soap and water, and you will get NO chemical bond, in any case. You'll
have to roughen the surface of the prime layer and depend on the mechanical
bond.
Second, "serious clamping" can be a mistake with epoxy. If it's squeezed out
to a layer thinner than roughly 0.003", strength falls off quickly. Below
0.002", it can all but disappear. Unlike resorcinol or some other adhesives,
you shouldn't clamp epoxy more than it takes to hold everything in place.
That applies especially to amine-cure epoxies. Clamping wood more than a
minimum will leave some percentage of the bonded area with a too-thin layer,
which can be a starting point for peel or cleavage failure.
As Paul also said, brittleness and low peel strength of most epoxies,
combined with swelling and shrinking of the wood, can lead to failure. Peel
strength usually isn't much of an issue when epoxy is used to bond wood, but
it's an inherent weakness of the adhesive -- again, especially with
amine-cure epoxies. It's most likely to show up at place where end grain is
exposed. Slowing down the absorption rate by coating with epoxy and fabric
is the best solution for that, because it will allow the creep rate of the
epoxy to partially catch up to the wood expansion, but it's not perfect.
--
Ed Huntress