The Rot Doctor


Subject: Caulking
Date: Wed, 05 Sep 2001

Good day, Dr. Rot.
My question concerns the caulking of a 36' wooden fishing boat which I'm seriously considering purchasing. She was built in 1951 and has relatively minor but persistent leaks at the bow and stern. At some point she'll have to come out of the water for repair (for my own piece of mind if for no other reason) and I'm wondering what you can tell me about caulking using the old iron and cotton method versus the newer caulking compounds. Should I use cotton first and fill in on top with a compound, or only resort to cotton if the gaps are large? The boat also has issues around the plywood cabin, but these are less of a worry than the hull leaks right now. I'm very new at the wooden boat restoration game and don't have deep pockets, so any advice you could offer would be greatly appreciated. Thanks.


The new caulking compounds do not exclude the use of cotton on carvel planked hulls. You must have the cotton in there, both as a backer for the compounds and as a "buffer" to absorb the swelling and contraction of the planks under different moisture/temperature conditions.

Old caulking compound should be pulled out from leaky areas and the cotton evaluated. If the cotton seems firmly in place and in reasonably good condition (that is, not deteriorated), then the seam sides can be carefully cleaned down to bare wood and a 100% polyurethane caulk applied.

If the cotton is missing or is deteriorated, then it should come out and new cotton applied. Using a mallet and a caulking iron is an art, I can tell you, and frankly is best done by a professional -- if you can find one. I've done it myself when I had to, but always wishing a better man was doing it. There are some books out there that describe the method, and the tools.

We believe all raw wood should have a coat of CPES (Clear Penetrating Epoxy Sealer) applied before finishes or caulks are applied. This is for protection purposes, and is not a requirement.

Examine the leaky areas and try and determine exactly where the water is coming in. This usually means laying on your belly with a bucket and a sponge and closely examining the seams.

Come on back if you have additional questions, or if you find suspicious wood. We'll help in any way we can.