I stumbled across some discussion about oak and epoxy
not getting along. Reports of the oak oozing a black/brown goo under
the plastic ruining the appearance and causing the bond to fail. So
far, I've found nothing conclusive and no clear consensus on the
subject. What I've found is some people are bonding oak and epoxy just
fine and others have had horrible experiences. I have epoxied over both
red oak and white oak very successfully, with no evidence of any
problems. If you plan on using oak, encourage you to do some research,
and test your material before bonding it into your valuable project.
Updated: 9/2006: My suspicion is that there is a
reaction from the tannin in the oak which reacts with the epoxy,
sometimes inhibiting the epoxy cure. It seems that if you add salt
water to the mix, it can get ugly I haven't seen it myself -- I've used
epoxy on oak on a couple of project since this was originally written,
all with good results.
I found a thread about oak and epoxy over at the kayak building
bulletin board at kayakforum.com
. Here's the pertinent text from West System tech support:
"Bonding to Oak is problematic, but these problems can be addressed
particularly in coating applications. I suggest you do a little
experimenting on a scrap piece before diving into your table."
"The structural issues with Oak won't apply to the table except as it
regards dimensional stability. Oak can and does move and twist quite a
bit more than other woods if it has a relatively high moisture content.
This may not apply to your table since it isn't fresh wood and it has
been indoors it's probably pretty dry, but you should determine this
before starting to work. Also, the epoxy will change the color of the
wood, even beyond the change from the amber cast of the 105/207 system.
We don't know why, but this is another good reason to do a test piece
first. Some folks don't like the appearance of epoxy coated Oak. As for
pre-treating, we have tried treating the Oak with a sodium hydroxide
solution to neutralize the tannic acid and measured the adhesion
against untreated wood and did not find a difference. This isn't to say
conclusively it doesn't matter because we didn't know the acidic pH of
the wood or the basic pH of the hydroxide solution, still the treatment
didn't help adhesion."
"To maximize the adhesion, sand the wood roughly in the direction of the
grain with 60 - 80 grit paper and use no solvents or tack cloths to
remove sanding dust. Vacuum and wipe with a damp paper towel. To
address the dimensional stability, if you do decide to coat the table, I
recommend you encapsulate the table top and bottom and sides. This
insures any movement of moisture in and out happens at an equal rate
and so stops twisting and cupping due to the relative humidity."
Temperature management during epoxy cure
Plastic molding was part of my job for over 20 years,
and I worked as an industrial patternmaker for more than a decade.
During that time, I did a lot of hand
fiberglass layup, working in epoxy, polyester, urethane, styrene and
other plastics, The applications were varied, often very different from
boats, but the glass was usually applied in much thicker sections, so
the conditions were even more sensitive to heat buildup than one or two
layers of 6-8oz.cloth like you'd use on a canoe or kayak.
To take a mold off a wooden part, if it was a cool day,
we'd gently heat the part with heat lamps to a bit over ambient room
temperature. Then the gel coat was applied and the temperature in the
room was slowly reduced to try to cause a very gradual cooling, to
prevent bubbles in the surface coat. If the weather was warm, we'd try
to time the first coat for a time of day when the temperature was
steady or dropping. (made for a lot of late nights!). Long experience
and many, many fiberglass layup jobs taught me the value of this
approach. If the internal temperature of the mold (or, in the case of a
strip-built boat, the wooden core, is allowed to increase while the
epoxy is still soft, or if direct heat is applied to the epoxy, air
that is contained within the cellular structure of the wood will expand
and create tiny bubbles.This does not refer to air in cavities due to
bad joints -- that's another, bigger problem that should be addressed
before applying epoxy. I'm speaking of nearly microscopic bubbles that
mar the surface and show up as little white dots.
I have seen numerous occasions where rising temperatures
during the cure cycle would cause pinhole bubbles in the surface coat,
and I suspect there are two primary causes:
I've heard arguments against this approach from people who's work I
greatly respect. Personally, I think their position comes from a lack
of experience, but that's just my opinion. I'm sure we could have a
huge discussion about what's actually going on -- but here's what I
think: If you keep the epoxy cool while the first coat cures, it stays
more viscous, and therefore resistant to forming bubbles. If you
maintain an even temperature in the room and the wood in the hull, you
reduce the pressure differential between the air in the hull and the
room. In a shop where fine temperature control may be difficult, an
easy way to do this is to allow the temperature to slowly drop, and to
keep heat sources away from the plastic so the hull cools at the same
rate as ambient room temperature. Add it all up -- more viscous plastic
to resist bubble formation, equal air pressure in the room and the
hull, or slightly lower pressure in the hull, and you have a condition
where it is less likely for air to be able to migrate from the hull to
the room (outgassing). ....or I could be totally wrong -- maybe it's
just that I always wear my lucky socks when I lay up glass. Whatever
the case, I have had consistently good results when using this
technique and a much higher percentage of failures and problems when I
didn't. ...so I'll keep doing it this way, and continue to recommend it
as an approach.
1.) Way too much heat applied
directly to the epoxy, like a heat lamp, will cause the epoxy cure too
fast, generate more heat and to form bubbles. This is pretty easy to
avoid by keeping heat sources away from the plastic, especially in a
thin coating like on a boat.
2.) gradually rising temperatures
inside the core (or the planking) causes the air in the porous wood to
expand slightly, and it generates pinholes as it finds a path to
escape. This is the much more subtle of the two problems, but can be
the most infuriating and seem the most random. On my boat, I only
worried a lot about heat on the inside hull surface, because with the
outer hull already sealed, there was no place for air to escape to
except through the wet plastic.
In both cases, the cure is to carefully monitor your temperature, try
to maintain an even temperature or to allow the workspace to gradually
cool while your first coat sets up, and to keep heat sources away from
the curing plastic. This is especially important if you are working in
a space where the temperature controls are not finely controlled, like
a garage or shed. Once the first coat is set up (you can leave a
fingerprint, but no plastic sticks to your finger), you have a lot more
latitude in temperature control, but you STILL want to avoid applying
direct heat to the epoxy.