Sheathing wood in Epoxy and wood rot

[Roach1948]

I was interested reading the article regarding Zeerand's (previously Merlin) restoration in the latest copy of Classic Boat. It seems from the article that she was splinned with epoxy cloth and then sheathed with epoxy. The owner quotes "The most important thing was to choose the right epoxy system, otherwise you end up with rotten wood pulp".

It seems this is a common view on the continent as I have a Swedish friend that runs a wooden boat yard who says if you sheath don't epoxy the interior and let the wood breath to stop the yacht rotting.

I am confused as there seems to be two approaches to sheathing; coating or not coating the interior. I was told by SP systems that if I were to sheath I needed less than 12% moisture content in the wood and that I MUST epoxy coat the interior. I also would need to spline the seems. The reason being that the hull would be a solid construction and any moisture coming in from the inside would make the wood move thus cracking the sheathing on the outside of the yacht.

I wonder which approach is correct?

 

[oldfrank]

Varnish or paint and sound wood is the best option for solid wood. Plywood may be different. If water gets in and has no exit your wood will rot - simple - applies equally to two pack poly as epoxy. OF.

 

[oldharry]

<<I wonder which approach is correct?>>

In a sense, both are right. Or rather, neither is actually wrong. Personally from past experience I would never sheath old plywood, as the moisture content is too high and the wood moves destroying the bond. Water then enters and your precious boat rapidly turns to pulp.

SP systems know their stuff, and if they say 12% moisture content is the maximum, then they are right. 12% is very very dry indeed. Freshly made good quality ply will be around 5% but it doesnt take long in damp storage for that figure to double or treble making sheathing impossible.

The SP system fully encapsulates the dry timber, so keeping it dry - until the sheath gets damaged! Dry wood will not develop rot, so it lasts indefinitely PROVIDED the owner is meticulous about repairing any damage before water gets in. Once it it will not come out again!

It is very unlikely that a used hull can be dried back to 12%. If you really MUSt sheath, then the damp timber will rot if you do not leave the interior able to dry out. But the end result is nothing like so durable as the SP system as the wood will move and the epoxy will not: almost invariably the wood will break at the wood/epoxy glue joint - allowing water ingress. Plywood is particularly prone to this type of failure, noteably where exposed end grains may already have started to deteriorate befoe the sheathing was applied.

I have owned a number fo wooden boats, and currently have a 26 year old plywood Eventide which was sheathed from new on the outside of the hull. At the last inspection the sheathed hull was as good as new. Only where an old botched repair to the topsides and cabin roof had allowed water to get in has there been any problem at all. Unfortunately the repair appears to have been done with Glass fibre and Polyester resin - an absolute no no for any wooden boat - so a largish section of the roof and cabin side is going to have to be replaced this winter.

As Oldfrank says, there is really no substitute for a good paint and varnish system on an older wooden boat whether ply or ordinary timber.

 

[Poignard]

Jubilee Sailing Trust's "Tenacious" was built of wood/epoxy. She is a few years old now and must have sailed tens of thousands of miles. It would be interesting to know how her hull is lasting. Anyone know?

 

[cliffordpope]

Wouldn't it make more sense to sheathe the inside?

 

[alb40]

The bottom of my boat has been epoxy sheated for over 10 years now, and there no problems down there. Inside is simply painted with red oxide paint.

The roof was a different story though - some idiot had sheathed it in polyester resin. It was so rotten that I could put my head through some of the holes that were under the fiberglass. I replaced the roof over the last couple of months and sheathed it in epoxy. Theres no paint or anything inside yet, but ill just give it a quick coat of flat white paint and put the headlining back up.

Sheathing both sides is not a good idea, the wood can at least breathe through the untreated side.

 

[cliffordpope]

Sorry, what I meant to say was, wouldn't it be more logical to sheathe only the inside? It's the outside that gets most wet, so it could continue to do so without trapping any moisture, but rain etc in the bilges would be isolated from the woodwork.
I appreciate that the framing and other obstructions might make this impracticable.

 

[alb40]

I think it is that the framing and so on would get in the way, and may it very impratical. Its also likely to keep rainwater that does leak in trapped, encouraging the rot to attack

It does have benefits on the outside though, like give good resistance to scrapes and bangs. On the bottom of the hull, it will also stop wood boring nasties getting into the wood etc.

The other big benefit i had last year was on relauch after 3 months out of water - didnt take on a drop of water!!

 

[johnchampion]

The decks and coachroof on my Tamarisk 24 are ply and have previously been sheathed with glass fibre and (I think) epoxy. This was presumably done when the boat was new however the sheathing on the side decks is splitting and coming adrift from the deck. Reading this thread the reason could well be movement and the resulting damp getting under the sheathing meaning delamination, more movement, more damp etc.
I had intended to resheath at least the side decks, again with glass and epoxy when she comes out for the winter after a few weeks under a tarpaulin cover, however, advice here seems to indicate this may not be the best solution as the ply is now some years old and no doubt will have an appreciable moisture content.
What difference though if paint/varnish or canvas and paint is used as the substrate will still be the same. I would have thought epoxy had more chance of staying in place than paint or varnish if the substrate is less than perfect, which of course at this stage it never will be. I also would worry if paint would stand up to the obviuos wear on decks.
I was idly thinking that if I used glass and epoxy I could happily forget it for ten years or so but by these comments maybe not.
What would our experts here consider the most appropriate approach?

 

[Lakesailor]

Whatever you do you need to dry out the wood. 2 or 3 months with air around it and protected from the damp. Then assess whether the ply is still sound. Otherwise I reckon you are wasting your time.

 

[Roach1948]

Many thanks for these views. As for the inside SP recommend a two part epoxy that is super-thin that can penetrate deeply into dry wood allowing the wood to breathe AND give protection at the same time.

 

[Roach1948]

Its called Eposeal 300 - not cheap. Has anybody had any experience of this product?

 

[carbon]

What sort of paint for the inside of an epoxy sheathed hull.will ordinary gloss Breathe?