Deck Seams Step-by-Step

Photos and Tips on Refinishing Deck Seams by Joel Terbrueggen

In my opinion deck seams of a boat are probably the most important aspect of a refinishing project. Everybody looks at the seams of a boat. Their appearance reflects the degree of workmanship that went into the restoration.

When laying new deck planks the seam is very easy – The width of the seam is the kerf of your table saw blade, depth about 1/8^th of an inch. Existing deck planks can be a far different issue. Often, I have seen bad seams due to cleaning out by every type of tool you can think of over the years – bent over file handles, screwdrivers, awls, etc. The following procedure has worked well for me over the years. 

– I prefer to strip the decks first. The edges are more visible

– A 3/32^nd up-spiral bit works best in a rotary tool, like a Dremel. A 1/8^th up-spiral can be used if the seams are wide. Using this tool can be a bit intimidating for some at first. I do not use a fence, just freehand pulling the tool toward me. I have found that old seam material is usually softer than the wood making this easier than you might think. 

– After cleaning out the seam with a shop vac or blow gun, use some sandpaper, I usually use 80 grit, folded over a putty knife blade to run through the seam. Do this to clean any residual material from the edges the tool left. 

– Proceed with the refinish/staining of the decks

– After the sealer is applied to the stain, apply 1 or 2 coats of varnish. This allows you to tape the edges without damaging the stain.

– Now tape off both sides of the seams. 

– Fill the seams with Mahogany colored Boat or Sikaflex 291 LOT Life (White on bleached decks, Black on dark decks). When applying the seam caulk, push the material to reduce chances of bubbles. If you do get bubbles immediately use a pin or something pointed to release the bubble before the caulk “sets”. My preferred method of applying the caulk is a battery powered caulking gun. You do not realize how many linear feet of deck seams you have until you begin the process of filling them. Applying constant pressure to a manual caulk gun is very hard on the hand (at least mine!). This can result in varying pressure due to cramping and produce an uneven flow of material. 

The Mahogany caulk blends into uneven seam edges that may have been created from past seam cleaning. I prefer to use my finger to force the caulk down into the seam while at the same time resulting in a nice concave seam.  Remove the tape after the deck seams are filled.

– To speed the curing, mist the decks with water.  

– Once cured to the touch, usually a day or 2, proceed with the varnishing.  

– After all varnish work is done, and before the hardware is applied, use a striping tool to paint the concave seams you created. My preferred striping tool is the Beugler Pinstriping Tool. The wheel I use the most is #73 which is 3/32” (2.3mm). The downside to this tool one is the cost. There are cheaper alternatives especially for the person that does very little seam striping. Also, I like to use an epoxy syringe to fill the tool. Less chance of a mess from pouring out of a can.  

– Plan ahead and tape any areas where the tool may roll paint onto the decks such as corners. Have some thinner ready on a rag.

– My seam paint of choice is One Shot. I find it is a little thicker than Brightside and dries much faster. However, if I do not have One Shot handy I have no qualms using Brightside Striping and Boottop.

– When you have long seams that require several segments always roll into the wet seam paint. If you try to start the second segment in a wet seam you will get a noticeable “glob” of paint. In other words, I am right handed and will always work my segments from left to right. On the Port side I start at the bow end and work toward the cockpit, on the Starboard side I start at the cockpit end and work toward the bow.

– Speaking of the cockpit, sometimes hidem is a factor. Use a thin artist brush to complete the final inch under the hidem edge.

– Sometimes a black Sharpie is all that is needed to color the mahogany Boat Life along the edge of a dark plank. It also can be used to color the whole seam if you didn’t have any black caulk. 

– In the future, you will really appreciate this method when it is time for maintenance coats of varnish. When you varnish the decks, the white seams turn yellow from the varnish. The seams will not need to be taped, just re-stripe with a striping tool since it already is concave. When seams are flat and need to be re-striped often they are taped. This invariably leads to a little wider seam each time varnish is applied. Extrapolate this over several maintenance coats and then you will have some VERY WIDE seams. I know because I have measured some ¼” wide. At that point, it almost needs a mini roller to re-stripe the seams.  

Joel Terbrueggen is a member of the Ohio Valley Chapter of ACBS. He has presented many restoration lessons which are available in the Video Library available on our ACBS Members Only site under the Resources tab.  Joel also donated his extensive library of cataloged photos taken as he performed surveys on antique and classic boats over a 16 year period.  These are also in the ACBS Members Only site under the Resources tab.  Joel Terbrueggen currently does much of the wood restoration on boats at the Motor Boat Garage in Cincinnati, Ohio.

Watch for more tips from Joel in the future.