Beltsander for fairing

Oak/Epoxy incompatibility?

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:
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.