Rot Doctor


Subject: CPES, Breathing (wood) and sealants
Date: Wed, 25 Oct 2000


I am truly confused. I am restoring a 14′ Stauter. I am almost finished removing all the old paint. You suggested 2 gallons of CPES for the boat. In one of your articles a person who had leaking seams on their sailboat was told to seal the outside with CPES but not to seal the inside so the boat could breath. Do I need to use CPES on the inside and outside of my boat?

The second question is do I need to seal all the seams/crack, for example where the mahogany ribs meet the light ply or where two pieces of light ply meet, where two pieces of wood come together. I observed that some of the seams/cracks had some type of sealant while others similar seams didn’t. I don’t know if I should seal them or what product to use.

The third question is what does breathing mean?

Thank you for your time and your advice.



You’re confused because it’s a somewhat confusing issue. With wood, “breathing” refers to the ability of the wood to pass air and moisture in and out, as temperature and moisture and humidity change. This is why a wood boat that generally sits in the water will open up her seams when she is hauled and left dry for a few weeks or longer. She’ll soak up again when put back in the water and the wood re-absorbs moisture, but not without leaking during the process. This is a bigger problem with planked boats than ply boats (more seams), and with larger boats than smaller boats (more wood to shrink/expand).

So, if you have a large planked boat and you wish to protect the wood, it is generally best to seal only the outside, leaving the inside wood bare so it will “breathe”. However, unlike most epoxy resin coatings, CPES is so thin that it soaks into the wood (rather than sitting on top of it) and doesn’t stop the “breathing”. It does inhibit it somewhat, but doesn’t prevent it altogether. Two coats of CPES inhibit the breathing more so, and three coats just about close off the wood, as a normal coating of thick epoxy will. Depending on the boat, its size, method of construction, intended use, etc. we will sometimes suggest that the CPES and other epoxies be kept off the inside. On other boats, ones that are smaller, kept on trailers or ashore, we will generally suggest that a coat of CPES on the inside will be a good thing.

Now, if there is wood deterioration/rot then all these considerations go out the window for that particular area. If you want to leave the wood in place and return it to useful service, then epoxy saturation with CPES and possibly Layup & Laminating Resin becomes essential. After this is completed, what is repaired for all intents and purposes is not wood any longer but epoxy resin absorbed into and around an old wood matrix. You can see a demonstration of this in our testing section where we combined CPES and L&L Resin with sawdust and balsa wood. It ends up being quite strong.

To answer your particular question, you can use CPES on the outside of your boat and if you wish you can safely use one coat on the inside. You boat is ply covered, so there will be less swelling/shrinking of the wood and “breathing” is less of an issue. However, if the inside shows no sign of being punky or soft, then the CPES is optional. By the way, all ordinary oil or latex based paints will allow the passage of air and water, so they do not stop wood from breathing. They can be applied over a coating of CPES very effectively.

Seams are sealed if you want to keep water out…otherwise, no, don’t seal them. On your boat the seams between the frames and the ply should not be sealed, because this will allow the wood to breathe. Wood that breathes cannot hold absorbed water (it evaporates away), and so it prevents the moisture level from being sustained at levels high enough to support the activity of fungi and bacteria. On areas where you want to keep the water out, generally on the exterior of the hull, then caulking/sealing is required. If the seam to be closed is on the hull itself, such as between hull pieces, which will be sanded and painted, it is generally best to use a hard, permanent seal, such as our Fill-It Epoxy Filler. If the seams are in places such as the hull/deck seams, then you will probably want to use a flexible sealant.

There are a bunch of sealants, and of these we believe the polyurethanes are the best. In the boating world, these are best represented by 3-M’s 5200 and 4200. The 5200 is a sealant/adhesive, and gives good bonding but less flexibility. The 4200 also gives decent bonding, and much better flexibility. Both of these adhere very strongly to CPES-treated wood. Never, ever use a silicone sealant! We don’t like them used anywhere, and on wood they are terrible because they contaminate the wood and prevent the bonding of epoxies and many paints, even after they have been scraped away.

Okay? This is a long answer, I know, but the questions you raise are good ones for the novice wood boat owner, and I wanted to be clear. Come on back if you need clarification or have other questions.