The Rot Doctor


Subject: CPES encapsulation of white cedar canoe planking before construction
Date: Mon, 19 Apr 1999

Doc Rot,

I'd like some info on brushing or spraying CPES onto 5/32" white cedar canoe planking before attaching the planking to the ribs with self-clinching canoe tacks.

The goals of pre-construction treatment with CPES are as follows:

1. Prevent moisture infiltration of planking when the canoe is finished and the canvas skin is attached (Linseed oil treatment and spar varnish are traditional coatings for both ribs and planking in a cedar/canvas canoe, but marginally effective as a cedar/canvas canoe may gain 10%-25% in weight on a wilderness trip where the boat is subject to frequent rain).

One coat of CPES will retard moisture infiltration, but not prevent it entirely. Two coats retard it even more, and with three coats you are starting to approach the point of epoxy encapsulation. This is not necessarily bad; it depends on what your intent is.

2. Prevent planking shrink/swell cycles.

See above: One coat will retard, two coats will retard more, and at three coats you are nearing waterproofness.

3. Add strength to the thin planking without making it brittle or significantly adding weight.

As above. CPES applied to new wood is not absorbed deeply (but more deeply than a standard, thick resin) and much depends on the porosity of the wood itself. It will add SOME strength, but usually if strength is an issue you need to add glass cloth.
Brittleness is not a problem with CPES on wood.

4. Prevent planking cracking from drying out if boat is left in storage for long periods.

Ah, this gets tricky. To prevent drying out entirely, you need to epoxy encapsulate with a thick resin, such as our Layup & Laminating Resin, on both sides of the hull. This of course means added weight and a different look. CPES will retard the drying/wetting process but not stop it entirely.

My personal opinion is that you do not want to stop the drying/humidifying process. It allows the wood to "breathe" and maintain its natural structure and integrity. In your case, most of the "breathing" would be done through the inside of the canoe, assuming that you applied only one coat of CPES there, which would be my recommendation. For endurance and appearance, if you wish, you could cover the CPES with a marine varnish, which will allow moisture to pass. CPES is a perfect base coat for any final finish.

The CPES would only be applied to the inner side of the planks prior to attachment (During planking attachment, the planks are sometimes heated with a damp rag and electric iron on the outer side to facilitate bending). After the hull is complete and finish-sanded, CPES would then be brushed on the outside of the planks to complete the encapsulation. The canoe would then be canvassed in the traditional manner.

This sounds just fine to me. I would only add that you pay particular attention to the plank seams and the end-grain. Make sure they are allowed to absorb all the CPES the can within about a 15 minute period of time.

Thanks Doc!

Gary P.


I do like your plan. I would do 1 coat of CPES on the inside and 2 coats on the outside before the canvas. This will give you some moisture protection, a little added strength, and good resistance to rot and bacteria problems.

Come back if you have more questions or need any clarification.

The Rot Doctor