Coating Wood With Epoxy to Prevent Rot

[cburger]

After having just built a new bowsprit for our boat a little over three years ago I discovered that sat the laminated Doug Fir bowsprit had almost rotted completely through and was pretty lucky not to have a failure of the rig. The old sprit was made from local lumber yard wood and was evidently wet and considering that I completely sealed the spar in epoxy as a water barrier coat it actually appears to have rotted from the inside out. I am now in the enviable position of making a new sprit and was able to procure a beautiful vertical grained old growth beam that came out of an old building rumored to have been cut in the mid 1800's.
I am trying to decide if I should coat the spar in epoxy, my instinct is this does not allow the wood to breath and may cause more problems than it solves. The other issue I have had issue with is paint does not seam to adhere properly my spars that are epoxy coated, even after dealing with the amine blush issues, am always going back and spot repairing and painting.

[minaret]

Originally Posted by cburger

After having just built a new bowsprit for our boat a little over three years ago I discovered that sat the laminated Doug Fir bowsprit had almost rotted completely through and was pretty lucky not to have a failure of the rig. The old sprit was made from local lumber yard wood and was evidently wet and considering that I completely sealed the spar in epoxy as a water barrier coat it actually appears to have rotted from the inside out. I am now in the enviable position of making a new sprit and was able to procure a beautiful vertical grained old growth beam that came out of an old building rumored to have been cut in the mid 1800's.
I am trying to decide if I should coat the spar in epoxy, my instinct is this does not allow the wood to breath and may cause more problems than it solves. The other issue I have had issue with is paint does not seam to adhere properly my spars that are epoxy coated, even after dealing with the amine blush issues, am always going back and spot repairing and painting.

You just need to finish with a high quality epoxy primer like 545. I apply sealer coats, then chemical bond build up coats to that. Then sand out the epoxy coats (usually 5-6) to 220 and spray on a few coats of 545. Sand it out to 400 and you're ready for topcoat that won't fail.


Kiln dried VG Doug Fir is no longer adequate for boat building. It needs to be quality quarter sawn sticker dried real Doug Fir, which is actually pretty hard to find. Your piece should be pretty nice.

[Cheechako]

You need to seal/paint the fir with something. It's not a real weather resistant wood, but fairly good. Old Growth should last a long time painted. I have seen too much epoxy blister and peel (on teak anyway) to want to do any more than a seal coat (to be soaked into the the wood grain) with it for outside stuff.

[sabray]

Coat the end grain of your finished piece until it won't soak up any more epoxy. Then go with prior advice. You are probably using better wood then what was original. Tight annual rings etc.... Bed the fittings real well. Maybe coat all the through bolt holes with epoxy as well. Seal it before you install and then do a finish coat.

[CharlieCobra]

What I've found over the years is that Penetrating epoxy doesn't penetrate very far at all across grain, only with it. Coating wood does no real good at all except at the ends.

[Roy M]

Dirty little secret: thin the first coat of epoxy with acetone or MEK to allow it to penetrate deeper. Then sand and continue as above.

[Stumble]

Fully encapsulating wood with epoxy is a pretty standard practice for preserving wooden parts. However there are a few tricks to get the most out of it.

1) never thin epoxy or used CPES. It is a myth that thinner epoxy penetrates noticeably deeper into wood than unthinned epoxies, and the thinners that are used dramatically decrease the structual integrity of the epoxy, and it's water proof ness. The best option is neat (unthickened) structual epoxies (West Systems is one example).

2) use multiple coats. Just like when painting, you get much better results with multiple thin coats than with one thick coat. You a re shooting for 400microns thick, which generally means 3 coats.

3) you need to start with a wood moisture content of <15%. This is because the threshold for rot is between 15 and 18%.

4) you must fully coat every bit of the wood. Inside the holes, kerfs, screw holes, ect... The better these small penetrations are coated the better the finished product will be.

5) because most epoxies have poor UV stability it is generally required that the coatings be protected from the sun, which of course means paint in most applications. I have had good luck with pretty much any commercially available epoxy based paint, but recommend marine suppliers just because of the added focus on UV protection.

Assuming you get full the wood fully encapsulated the piece will become isolated from the outside environment, meaning it becomes dimensionally stable (no humidity swelling), and the epoxy itself adds stiffness to the piece (generally a good thing, but there are exceptions).

[rebel heart]

Originally Posted by CharlieCobra

What I've found over the years is that Penetrating epoxy doesn't penetrate very far at all across grain, only with it.

I'd agree with that, although the idea that it does no good to coat in general I don't believe. Epoxy is a very tough barrier and even if paint splits there's a chance that the epoxy underneath is still holding.

To the thread starter, I'd focus on where hardware goes into the sprit. I have a wooden bowsprit myself (17' long) and it's always where the hardware goes in that water penetrates. You *really* want to overbore, epoxy, then drill into the epoxy for any fasteners. It's a pain in the ass and takes forever but it's the only reliable way to keep water out.

Water isn't going to find it's way through a painted flat piece of wood, it's going to work it's way through a fastener hole.

[Pauls]

There's not much mystery about what it takes to give wood a long life on boats. Any marina will have some old wood boats with many many pieces that are quite old and still sound.

The exposed parts of the wood will be pretty rot resistant, and if sealed with any of a large variety of sealers will stand up well. Refer to old wood sided houses, they're out in the sun and rain and last a long time.

The areas that require careful attention are where moisture can be trapped and the wood can't breathe. Your cranse iron, the butt cap if you have one, holes for fasteners, etc are where the trouble will start. For these, do everything you can to seal the wood, end grain in particular should have sealer applied until it will not soak up any more. I don't honestly know if epoxy is any better for this than other products. I've built large wooden boats and am something of an adhesives expert for my industry and I can tell you for sure that neither epoxy or any other plastic type material is waterproof. Water in its gaseous form penetrates epoxy quite readily, and that gaseous water can and will condense back into liquid after it has penetrated. We measure this, it is not theoretical. My completely unscientific quess (and it's unscientific because I've never done any testing to measure water absorption of end grain wood treated with different sealers) is that well thinned polyurethane varnish is probably the best sealer, just because it has a lower viscosity than any thinned epoxy and so I'd expect it to wick further into the end grain. I've used this some and you will need to apply a number of coats with a good drying time in between to allow the solvents to evaporate.

Anyhow, seal the wood where fittings attach the best way you can and then seal the fittings against the wood with a high quality, flexible sealer to form a second barrier against water penetration. I definitely like setting screws into epoxy filled holes.

Regards, Paul

[Roy M]

Stumble, I've been using West System for approaching forty years. Gougeon Brothers, the developers of the West System, offered me this tip many years ago when I was repairing a tricky repair of someone's dry rot. So, before offering your advice as to NEVER do something, as an attorney, cut yourself some wiggle room, you may not know everything about this subject. Your other advice, though, was pretty correct, with the exclusion of epoxy paint in sunlight. Those paints tend to begin to break down and "chalk" pretty quickly in UV. Save epoxy paint for interior applications. Linear polyurethane is a much better bet for longevity and investment.

[Delancey]

Originally Posted by Stumble

Fully encapsulating wood with epoxy is a pretty standard practice for preserving wooden parts. However there are a few tricks to get the most out of it.

1) never thin epoxy or used CPES. It is a myth that thinner epoxy penetrates noticeably deeper into wood than unthinned epoxies, and the thinners that are used dramatically decrease the structual integrity of the epoxy, and it's water proof ness. The best option is neat (unthickened) structual epoxies (West Systems is one example).

2) use multiple coats. Just like when painting, you get much better results with multiple thin coats than with one thick coat. You a re shooting for 400microns thick, which generally means 3 coats.

3) you need to start with a wood moisture content of <15%. This is because the threshold for rot is between 15 and 18%.

4) you must fully coat every bit of the wood. Inside the holes, kerfs, screw holes, ect... The better these small penetrations are coated the better the finished product will be.

5) because most epoxies have poor UV stability it is generally required that the coatings be protected from the sun, which of course means paint in most applications. I have had good luck with pretty much any commercially available epoxy based paint, but recommend marine suppliers just because of the added focus on UV protection.

Assuming you get full the wood fully encapsulated the piece will become isolated from the outside environment, meaning it becomes dimensionally stable (no humidity swelling), and the epoxy itself adds stiffness to the piece (generally a good thing, but there are exceptions).

I think Stumble did pretty good job except for one part, The Gougeon Brothers recommend against trying to epoxy encapsulate dimensional lumber thicker than about 3/8" thick.

So if you want to encapsulate a larger piece they recommend it be made by laminating thinner pieces.

I think this is based on the notion that if the wood does move, the amount it will move cumulatively on larger pieces will exceed the elastic limit of the epoxy and cause it to fail.

So in this case they would recommend against trying to encapsulate the bow sprit. Maybe try a good quality paint or varnish and stay on top of maintenance?

Everything you ever wanted to know about wood epoxy construction in the link below-
http://www.westsystem.com/ss/assets/...k%20061205.pdf

[Sand crab]

Roy M is mostly correct about epoxy paints degrading in sunlight. However there are new epoxy paints that specifically claim they are UV stable. At least they say they are. Anybody try these?

[hellosailor]

Epoxy penetrants may be a myth but I've seen GitRot, an epoxy penetrant, go far deeper into rotted wood than epoxy could.

If you have a slow-curing thin penetrant, you can also pressure treat the wood with it. Seal it in a bag with excess material, tie a good ine and weight to it and drop it 50-100 feet into the water. At that point it is under 2-3 atmospheres of pressure and yes, that forces the penetrant deeper into the wood. (This can be done with wood preservatives, not just epoxy.)

Then it is just a simple exercise to haul it back up and clean the excess off before it hardens up.

[Stumble]

Delancy,

I haven't seen that recomendation but I can certainly see the logic in it.


Roy,

That may have been the

Recomendation years ago, but it certainly isn't today. Solvent thinning epoxy has been shown to cause the resulting epoxy layer to suffer from micro fractures caused by the solvent evaporating. These fractures then act as entry paths for water vapor.

Moisture uptake (% by weight) at six weeks 100% humidity
From Thinning West System Epoxy

33.66 - 4 coats solvent thinned epoxy
7.57 - 3 coats West Systems
7.24 - 2 coats solvent thinned then 3 coats West
5.76 - 4 coats West Systems
3.43 - 7 coats West Systems

As you can see using solvent thinned epoxies has almost no effect on the permeability of water to the underlying wood. Two starter coats of solvent thinned epoxy on increases the permeability by .33% while an additional coat of neat West Systems adds 1.81%, and four coats of thinned epoxy is 27.9% worse than four coats of neat epoxy.

[hellosailor]

Ah, no, the plastic bag doesn't have to enter the wood. It compresses all the fluid (and gas) in the bag against the porous wood, and since the fluid is incompressible, any gas in the wood is compressed and fluid impregnates it. Basic hydraulics, no magic.