QUESTION RE: Drying and movement of Epoxy/Resin "Encased" Wood?

[mredden]

GENERAL QUESTION: What does wood do when totally sealed inside marine application epoxy resin?

Background: My client wants his table now (don't they all?). I am planning to build it from a large pecan blowdown that I cs milled from a log on his property. It's going to be an epoxy/resin river table.

The wood has been air dried and is down to about 14% now. That is about the average annual EMC for this area. It is to be used on a porch that gets sunlight each morning until about noon. I am considering joining and sealing it thoroughly with a marine application epoxy resin and an overcoat of spar varnish for UV protection. It will get little (but some) rain but constant high humidity.

Although frequently touted as impermeable to water, my limited research shows that some small amount of moisture can pass through epoxy resin. So, should I have any concerns about the wood passing out or taking on moisture?

*Since I am at EMC or thereabouts, has the wood ceased to move, or will it continue to SHRINK, expand, twist, cup, bow, etc. after flattening, finishing, joining and sealing or will it be "fossilized"?

* Non specific to me, but curious: what would happen if you encased wood that was well above EMC - say 25%?

*

[Old Greenhorn]

Great question. I can't help you, but I will be watching this for the discussion that ensues. I have wondered about this myself. In fact, I made a small bench at about 20%MC and gifted it to a neighbor with the understanding that I could check on it from time to time and watch for changes. I also made a bar back in September that was at 12% (ambient for here) and will be watching that for changes also. Both are finished with Verathane. Lets see wheat the smart folks have to add.

[DWyatt]

@tule peak timber will probably be the guy to talk to about this. He's forgotten more than most people will ever know about epoxy.

[mredden]

It appears that the Gougeon Brothers (experts on boat construction) recommend a moisture content lof 12% for proper bonding of epoxy resin with wood. My wood is below that level at the surface up to 1/4 inch deep  - but more like 14-15% at 3/4" to 1" depth. I think I'm okay here on the  bonding, but somebody let me know if my logic is screwy.

[doc henderson]

if it is at steady state for your area outside and stays outside, it should be ok.  I think higher moisture (>25%)could cloud the epoxy.  after cured should be ok unless someone decides to take into the air conditioned/heated house then it's any ones guess

[Old Greenhorn]

........ but somebody let me know if my logic is screwy.

Well you hang out with people like us so...... ;D :D
 Sorry I could not resist. Sometimes there is straight line hanging out there just begging to be swung at.

[GeneWengert-WoodDoc]

Epoxy is very sensitive to degradation by uv light, unless a special type of uv-stabilized epoxy is used.

I think you plan to coat the entire table with epoxy.  The coating must be thick enough to generate the heat required for the epoxy to cure.  (Special note:  Glue joints with epoxy are often squeezed too much, so that the epoxy never cures properly.  Then people wonder why the epoxy joint fails at low stress levels.)

Epoxy does act as a very good moisture barrier, so moisture changes in the wood will be slow if the top, bottom, and edges are coated well.  Slow, small moisture changes can usually be well absorbed by the wood.  However, any small cracks do have a risk of opening slightly more.  Chances are good you will have a major crack problem if this very high movement wood is brought inside and allowed to dry to 5 or 6% MC.

[GeneWengert-WoodDoc]

One additional comment is that epoxy coatings are fairly hard, but will scratch and not appear as clear as they were originally.  Also, they yellow with exposure to ultraviolet light.  For this reason, many table tops are coated with a different polymer such as acrylic.  Acrylic can be polished and kept clear, if it scratches.  As with any coating on a large piece of wood, coat top and bottom to maximize stability with moisture changes.  Acrylic is used for windshields on small aircraft.  Plexiglass is a trade name..

[Stephen1]

I have sawn wood for a young artist and his father. He started epoxing 10 and 12/4 slabs green  right off the mill. His theory is it will encase it and it will not move .
I'm waiting to hear how they turned out.

[mredden]

Epoxy is very sensitive to degradation by uv light, unless a special type of uv-stabilized epoxy is used.

I think you plan to coat the entire table with epoxy.  The coating must be thick enough to generate the heat required for the epoxy to cure.  (Special note:  Glue joints with epoxy are often squeezed too much, so that the epoxy never cures properly.  Then people wonder why the epoxy joint fails at low stress levels.)

Epoxy does act as a very good moisture barrier, so moisture changes in the wood will be slow if the top, bottom, and edges are coated well.  Slow, small moisture changes can usually be well absorbed by the wood.  However, any small cracks do have a risk of opening slightly more.  Chances are good you will have a major crack problem if this very high movement wood is brought inside and allowed to dry to 5 or 6% MC.

The things I have read from boat makers indicate that epoxy resin surfaces can/should be coated in spar varnish to protect against UV light. Of course, even spar varnish will eventually break down after years of exposure to sunlight, but it can be sanded off and reapplied every few years.
Not sure how long that would be with direct light exposure every morning until just before noon, but wooden boats protected by spar varnish seem to do well
.

[GeneWengert-WoodDoc]

Spar varnish absorbs some ultraviolet, especially if a uv absorber is added.  It is better to get uv-stabilized epoxy.

[Don P]

If there is a gradient the wood is not done moving. Wood draws unto itself as it dries. Checks and glueup edges change shape, usually opening up more as they season. If someone insists that their schedule is more important than nature's all one can do is educate them, make it clear they are driving, and wish them the best. Don't let him make it your fault later. I suspect the finish will crack in places and the piece will distort to some degree. If it is unacceptable to him at that point you can redo it again later. There is less movement starting from the dry side and coming back up to emc than coming down to it, hysteresis.

[GeneWengert-WoodDoc]

If you encase a piece of wet wood in a perfect seal, then no moisture can get out and no air can get in.  However, if there are a few insects or fungi in the wood when encapsulated, they can live for a while, possibly weakening the wood.

If the epoxy is not a perfect seal, maybe from holes that hold the legs, or a poor coating in a spot, or mechanical damage that cracked the coating, the wood will dry and try to shrink.  This can cause cupping or other warp and cause honeycomb in the pieces, weakening the product.

[WDH]

If someone insists that their schedule is more important than nature's all one can do is educate them, make it clear they are driving, and wish them the best.

That is a profound statement. 

[Old Greenhorn]

Some folks prefer to pay for their education in real time. ;D

[GeneWengert-WoodDoc]

The moisture content of wood in a boat will be over 18% MC average...in foggy, rainy  weather, closer to 28% MC and in sunny weather closer to 16% MC.  So, 20% MC is a good target for them. 

 However, epoxy will work very well at a wide range of MCs...6% MC to 25% MC.  However, avoid large moisture changes after application of the epoxy if at all possible.  And make sure it is applied thick enough, as mentioned earlier.  Also, consider adding very fine sawdust as an extender...use less epoxy but get the same results on most surface, but probably not on the top as the dust does reduce clarity a bit.