The Rot Doctor


Subject: The Epoxy on Wooden Boat Wars
Date: Sun, 18 Jun 2000

I've been reading up on the many forums and websites dedicated to wood constructed vessels and epoxy on wood hulls has its allies and its enemies..I'm about to purchase a 36' 1969 Trojan Sea Voyager(woody)..the hull is in good shape and ready for painting..I'm thinking about using your CPES to seal it before painting but I have a question..wood fibers naturally expand and contract as you take it to and from the water..wouldn't this process tend to separate your resin and make it useless?..I've read your previous responses and you point out that it has some elasticity but does it have enough?




Good question...and you're right about the wide variety of opinions on using resins on wood hulls. Way back in the '70's when the resins began to appear on the store shelves, old boat owners thought they might be the final answer to those old leaking hulls. In most cases they weren't and the hulls went to the bottom pretty fast.

We're much more sophisticated now and the resins, especially epoxy, are widely used on wooden boats. The usual caution has to do with enclosing the wood so that it can't "breathe", or if it can breathe it will expand and contract under the resin sheathing and cause problems. These are genuine concerns, but are somewhat dependent on the size and type of hull. For large planked hulls it makes the use of resins almost impossible unless the hull is hauled, seams reefed out, wood thoroughly dried, and then a whole new hull basically constructed using the old hull as a mold. Can be done, but is time-consuming and expensive. On some of the smaller ply hulls the judicious use of resins and glass sheathing has worked -- the ply is easier to dry and has much less expansion/contraction.

On particular areas being repaired on wooden hulls, these concerns are generally not an issue. Epoxy can be used in local repairs and there is enough of the wood left to allow the normal expansions and contractions of the wood without affecting the areas that have the epoxy application.

Now, about CPES. CPES is a premium epoxy that is carried into the wood by a mix of 14 different solvents, moisture displacers, sap dissolvers and the like. When mixed it is as thin as diesel fuel. It penetrates into any gaps in wood, concrete, fiberglass, etc., the carrier solvents evaporate away, and the resin remains, binding things together and forming a semi-waterproof barrier. Wood that has been treated with one coat of CPES will still "breathe", although a bit more slowly than untreated wood. Two coats of CPES will still allow air and moisture to pass, although more slowly still. And three coats of CPES begin to seal things up, although still not totally. So the bottom line here is that CPES gets between and around the viable wood fibers (or inside the damaged fibers), hardens things a bit, and prohibits the wood from absorbing as much moisture as it normally would. It also creates an epoxy surface environment that many fungi and bacteria would prefer not to eat. When applied to the end-grain of wood, quite a lot of CPES can be absorbed to very good effect, because it's here where wood deterioration problems often get started.

So, CPES can be and is applied to good wood to give the thin protective barrier that discourages the development of rot and bacterial degradation and forms a very superior base-coat for subsequent coatings. Because CPES is so thin, it does not "layer-over" the wood with a waterproof resin, and so the wood can still expand and contract, although as noted above a bit more slowly than un-treated wood. If you've got a leaky old hull that you're going to re-launch, then it's just going to take it a little longer to swell herself back up.

Our testing has shown that when paints/varnishes are applied over CPES-treated surfaces the life of the paint/varnish is extended substantially. We also know that when sealants, such as 3-M's 5200, are applied to a CPES-treated hull before the CPES has completely cured (between 20 and 60 minutes at 70F -- shorter if its hotter, longer if its cooler) the bonding is stronger than it is to un-treated wood. In the case of 5200 this bond is almost unbreakable. The same addition of bonding strength applies to 4200, 101 and the polysulfides.

In your particular case, I would say that CPES applied above the waterline would serve as a very good wood protector and primer for subsequent paints. Below the waterline it might or might not be beneficial, depending on whether the boat stays in the water or is hauled. If it's hauled seasonally then The CPES would be very helpful. Expansion and contraction of the hull will not be a problem. The coating is elastic enough and thin enough that you'll have no worries. Many do this, and many are happy.

This probably more than answers your question, but if you have additional questions then come on back.