Caulking for Small Seams

30 Jul 24:

Images of caulking a seam often include specialized mallets and large caulking tools called irons, used to push large strands of cotton or aokum into an open seam between planks. Small boats have small seams, hopefully tight fit, so the tools used to caulk a seam are smaller. As the seam width gets smaller, so does the thickness of the caulking iron. ANd to avoid damaging a thin plank, very little force is used to force cotton into a seam. In the case of this seam, it is the perfect width for us to us a seam roller to ease the cotton into the seam.  The iron used has a thickness of 1/16th inch, and the cotton strand is about 1/8th of a full strand. For any tighter of a seam we'll switch to cotton wicking, which is similar to a thick candle wick.

Caulking a Small Boat - Tools, Tips and Materials

12 May 24:

Caulking cotton and wicking are used to fill seams between planks. When water finds the cotton, both the cotton and wood swell to create a watertight seam. A one pound roll of caulking cotton = 210 feet. We ordered from Defender but Jamestown Distributors is our usual source. We also ordered caulking wicking. We used Defender because we wanted to try the Teak Decking System seam sander and reefing knife, but found out that these tools are way too big for the tiny seams on our dinghy.

One tip with the cotton is to roll it into a ball and keep the ball in a clean container. This keeps the cotton from picking up sawdust, dirt, twigs etc... from the shop floor and other tools can ride around in the box as we work our way around the seams.

Different seam sizes = different materials. Wicking and cotton can be either peeled apart to get the right size for smaller seams or doubled up for larger seams. For this thin 3/8" planking we used about 1/3 of a strand for a 1/8" seam, and less for the tighter seams. When the seams got real tight we switched to wicking. 

We have a traditional caulking hammer, irons and seam roller. For light boats and small planks a caulking hammer can provide very light taps to the metal irons. Any other type of mallet or hammer can be used, but it is best to strike the metal irons with wood or nylon, as a metal hammer would wear out the head of the caulking irons and it is noisy. 

We are not sure what this tool was originally designed for, leather work possibly, but it makes a great tool to roll half a strand of cotton into a 1/16" seam.

Video: https://youtu.be/BNg4X5PO5hM

We broke out the TotalBoat TotalFair to cover the rivets heads. Some folks do not fair over the heads, but they had been faired over in HENNING, so that tradition continues. 

Once the cotton was in the seams, we put a protective coat of primer over the cotton from George Kirby Jr. Paint Company. If your wood is very dry and/or new, it may help to seal up the seams with a wood sealer or primer before caulking. Then either wait for those products to dry or plan on having a messy caulking experience. 

This side is caulked and faired.

Still need to caulk this side but the rivet heads are faired. 

Next up? Sand the TotalFair then get more primer on the wood. Following that we will put a flexible compound in the seam to protect the cotton and that will move a little as planks shrink and swell. And paint. 

Log of HENNING.

Wooden Boat Back Issues on Caulking and Pete Culler

31 Dec 24:

A few excerpts from past issues of WoodenBoat Magazine, about to enter its 50th Year of Publication. Great tips on caulking and a look around Captain R. D. "Pete" Culler's shop.

Barbashela Log 18 Jun 16

18 Jun 16:

Caulked the planks with 1 pound of Bristol Sussex cotton, also a little wicking in some areas. Painted the caulked seams with alkyd enamel (oil) to seal the cotton.

Cleaned out the starboard garboard seam and filed down corroded areas for dutchmen.

Posed for pictures.

Click her for Barbashela's full story.