How to Fill the Gaps in a Dovetail Joint

The best solution is to remake the pin

Everyone would like to cut perfect dovetails, but there are very few who actually do. I am not one of those hallowed few. So, I’ve learned how to fix the gaps in my joints. I’ve also learned how not to fix them, and that many do not need to be fixed at all. Let’s start there with the ones that do not need filling.

There always seems to be at least one small gap in a set of dovetail joints. They show up as thin shadow lines and are quickly filled with sawdust when you sand the joint. You do not need to fill them. No one will notice them unless you point them out, which I know you really want to do, but please do not. Mistakes are part of learning a craft, and you should not be ashamed of them. They mark your growth as a furniture maker.

Still, if you insist on filling them, for the love of Sam Maloof do not make a gooey slurry from wood glue and sanding dust and then jam it into the sliver of a gap. Nor should you use store-bought wood filler. Instead, sand the joint to fill it with dust. Then put some shellac over the gap and let it soak down into the gap as it’s absorbed by the dust already there. Sand the joint again while the shellac is still wet to make sure the gap is full of dust and shellac. Give the shellac some time to dry and sand again. The dried shellac will hold the dust in the gap, and sanding the surface removes any shellac still there, so it will not affect whatever finish you eventually apply. Use the clearest shellac you can because the more color the shellac has, the more noticeable it will be in the filled gap.

OK, let’s talk about the gaps that really need repair, like the one in the photo below, between the tail on the right and the pin on the edge of the pin board.

This gap is too large to fill with dust and shellac. An oft-suggested fix for gaps such as this one is to make a wedge from the same wood as the pin board, slap some glue on it, and then drive it into the gap. It works but at a cost. It’s a wedge, so it will absolutely push the edge of the tail away as you knock it home. You end up with a tail that’s noticeable “off slope.” And the end grain of the wedge probably will not match the grain on the pin. That’s noticeable, too, and looks bad.

Here’s how I fix gaps like this one. I glue a patch onto the narrow pin and recut it. First, I grab the offcut left over after I cut the pin board to length. Yes, you should save those offcuts until after the joint has been fit, glued together, and cleaned up. I examine the end grain and look for a little spot where its grain is a good match for the grain on the pin board. Cut a “patch” from there. It’s critical that you cut the patch the correct way. See the illustration below.

If you cut the patch with two cuts square to the board’s face, you’ll have to rotate the patch a bit when you glue it to the narrow pin. This also rotates the grain, and the grain on the patch will run in a different direction than the grain on the pin board. That will be noticeable, and a fix that’s noticeable is worse than a gap.

When you cut the patch, the side that is glued to the narrow pin should be angled to match the pin’s slope. This has two advantages. First, the grain on the patch will match the flow of the grain on the pin. Second, the side opposite of the glue joint is parallel to the edge of the pin board, and that makes it easy to clamp while the glue dries.

I make the patch plenty wide enough for me to lay the tailboard back down and transfer the tail again. After transferring the tail, I recut the pin and fit it. The results are pretty much seamless.

Well, that’s it. That’s how I fill gaps in my dovetails. I know it’s a somewhat tedious and time consuming process, but it’s better than a big gap and way, way better than a slapdash repair.

I am always here to read your comments and questions. Please give the post a like and share it. Thanks for reading, and have a great weekend!