Several Ways to Fill Gaps in Wood

Ways to Fill Gaps

Fine woodworking requires considerable accuracy and finesse, often as much as that which is produced by a machinist. Yet the woodworker, especially the do-it-yourself woodworker, works with equipment which doesn’t have the inherent accuracy of the metalworking mills and lathes the machinist works with. Rather, the woodworker has to bring that accuracy to the workbench through their own skills and a discerning eye.

One of the more frustrating aspects of doing home woodworking projects comes in the form of a mismatched joint where the wood isn’t seamlessly joined together. Tight, seamless joints are the defining characteristic of quality woodworking, especially in furniture, and many a woodworker has despaired of accomplishing them.

It is easy to blame one’s tools, when joints don’t turn out as tight as we would like. But not all the blame rests there. If we look at the workmanship of woodworkers in the Middle Ages, we can see many examples of exquisite woodworking, all accomplished with much more rustic tools than we have available to us today. And they did it all without power tools.

Poor joints not only frustrate the woodworker, but they speak of the quality of his work. Anyone looking at the finished project will naturally find their eye drawn to the poor joinery work, making the skill level of the woodworker apparent to all who see it.

Obviously, any of us would like to be able to eliminate these gaps in our projects and have everything we produce look perfect. Yet everyone who makes furniture runs into this situation at some point. Nevertheless there are a number of ways of fixing these gaps in our projects’ seams, including: glue and sawdust (which includes epoxy/sawdust mixtures), commercially available wood filler, and using a small sliver of the actual wood.

sawdust, packed, plastic bags, wood, oak
Sawdust packed in plastic bags, Joel Washing

Wood fillers

The most common means of repairing any flaw in a woodworking project is with commercially made wood fillers. These fillers are manufactured by a number of the companies which provide wood finishing products and adhesives. They come in two basic styles: “stainable” and tinted.

Stainable wood fillers

Stainable wood fillers are a bit of a misnomer. While they all claim to accept stain, they don’t do it as well as the wood they are filling. Some do it better than others, but I have yet to see any “stainable” wood filler that will actually accept stain to the point where it really matches the wood that it is being used with. At the best, you can expect these products to come close; but they’ll never be exact.

When using these wood fillers, you must expect to apply additional coats of the stain directly onto the wood filler, after staining the wood. That will allow you to darken the wood filler, more closely approximating the color of the stained wood.

While these stainable wood fillers don’t work all that good when it comes to matching the color of wood grain, they are excellent for use in painted wood projects. The wood fillers will nicely fill seams and nail holes, giving you a smooth, seamless surface. Since they shrink when drying, excess should always be applied, and then sanded off smooth with the surface of the wood, once dry.

Pre-tinted wood fillers

Pre-tinted dedicated wood filler provides you with wood filler which is already tinted to match common wood colors. Available in a variety of styles and colors, wood fillers offer an attractive alternative to the approach of trying to create your own flour paste to fill your gaps. Many woodworking supply companies carry these wood fillers, colored to match many of the common hardwood species native to the US.

These fillers are made from real sawdust from the species on the label, and a can of this material (6 oz., 170 g) runs about $6 USD (~€5). These products take some of the guesswork out of trying to make your own wood filler, but unless you have a cabinet filled with the various colors available or purchased a can just for the project you are working on, using these often means allowing your project to grind to a halt, as you head to the home-center or (worse) you wait for it to come in the mail.

Nevertheless, these wood fillers aren’t a perfect match. Anyone who has done woodworking for even a little while, knows that wood coloration varies, even within the same tree species. On top of that, most woods have two primary grain colors, but can, in reality, have as many as six. So, just filling a seam with one of these premade wood fillers may not provide you with an invisible seam.

Working with wood fillers

There are two solutions to this problem; both of which can and should be used concurrently. The first is to mix colors of the same brand together, so that you can come up with an exact match for your wood. Since your wood has several different tones in it, the second is to not mix just one color, but mix several. Then, when you are filling in nail holes, seams or other gaps, you can use a combination of different putty colors, hiding the seam better than one color alone can do.

Ideally, the wood filler should be a putty-like consistency. However, you may find that your wood filler isn’t that thick. If that’s the case, it’s really not a problem. You can thicken it by mixing “painter’s whiting” into it. If you can’t find painter’s whiting at your local building materials center, it is nothing more than white chalk dust. You can easily make your by filing the inner part of drywall with a surform rasp or by using a normal rasp on a stick of white chalk.

To mix the painter’s whiting into your wood filler, spread out some of the whiting on your workbench, a paint can top or an old plate. Then take a ball of the wood filler and press it onto the whiting, allowing it to stick. Knead the ball of putty, working the whiting into it. As you work, you will notice that the more whiting you work into your putty, the thicker and stiffer it gets.

In order to fill gaps or nail holes with the thickened putty, pull out a corner and turn it between your thumb and forefinger, making a thin round protrusion. Press this into the seam or hole, brushing it sideways with your thumb to wipe off the excess. Save the leftovers in an airtight jar for the next time.

This method works extremely well when working to fill nail holes in stained and varnished architectural wood trim. You’ll want to fill them after staining and applying at least one coat of varnish, but before applying the last coat of varnish. It is not necessary to wait for the filler to dry, before varnishing.

Speaking of chalk dust

Chalk dust can also be used in another way, to make your own wood putty. This is ideal for use in filling nail holes in painted wood trim and other painted projects. One of the great things about it is that it uses paint as one of the two ingredients. Therefore, if the paint that is used to paint the project is used for the putty, it will match, without having to paint over it.

In order to make this you’ll either need the same painter’s whiting that I was talking about a few paragraphs back or chalk dust. If you don’t have either, the easiest way to get chalk dust is to grind up the core of a piece of plywood, with a surform file or food grater. A blender or food processor would work as well, but you’d want to clean it up well, before the cook in the family finds out.

Make a small mound of the chalk dust on a non-stick surface, and then drip a few drops of the paint into the middle of it. Starting with a putty knife, mix the two ingredients together. Add more chalk dust and/or more paint as needed to reach a putty-like consistency. Once you get to that point, scrape the putty off of the surface you’ve been mixing it on and knead it in your fingers. If you can knead it in your hands, it’s ready to use.

This putty is used in the same way mentioned above for using pre-tinted putty that has been thickened up with chalk dust. Be sure to store it in an airtight container, if you expect to use it again.

Sawdust and glue

Experienced woodworkers don’t always bother to buy wood filler to match their projects. Instead, they use sawdust from the project itself and wood glue. For the most part, if the seam is small (< 2mm, 1/16th of an inch) or in an out of the way location, using a mixture of sawdust and glue should not only cover the seam, but if the ratio of glue/sawdust has enough sawdust in it, the resulting finish should hide the open seam very well.

Some woodworkers don’t care to use this filler method as it often results in the filler being much darker than the surrounding wood once the finish is applied. This darkening of the finish comes from the tremendous amount of surface area associated with sawdust “flour” generated from table saws and other woodworking machines (routers, bandsaws, etc.).

However, if your sawdust also includes shavings or larger diameter dust, you can create a dust/glue mixture that won’t absorb as much of the stain/finish and the resulting applied finish won’t be darker than the surrounding wood, making your results will look much better as the lowered surface area of the shavings/larger dust lowers the amount of stain absorbed by the repair mixture. If your dust only includes the ‘flour’ type of sawdust created by tablesaws, then mixing this flour with items that don’t absorb stain (chalk—calcium carbonate or gypsum from drywall) can improve the absorption characteristics of the mixture. Also, adding a little extra glue to this mixture also reduces the amount of stain the filling absorbs. Glue doesn’t take stain, as any woodworker knows since we’ve ALL made the mistake of not wiping away all of the glue from a drip, only to witness the horror of a section of your project that doesn’t take any color whatsoever because of that excess glue.

Or shellac

The same thing can be done using clear shellac, rather than using wood glue. There are two advantages of using shellac, rather than wood glue. The first is that it will match the color of your workpiece even better, especially if you use sawdust from your project. The second is that shellac dries very quickly, much more quickly than wood glue does. If you’re filling a vertical surface, that’s important.

Be sure to buy clear shellac for this, rather than naturally colored. Naturally colored shellac is orange, not clear, so it will affect the finished color of your project.

When mixing, you’re looking for a consistency something like cornbread, just shy of being able to be molded into a ball. If you can stick it together in that ball, you’ve probably got too much shellac; add some more sawdust to make it a bit drier.

Using a piece of the wood

Perhaps the most effective, but hardest, method of fixing small gaps in wood is actually using a piece of the wood from the project itself as the wood filler. The benefits of this approach are obvious; the wood absorbs stain/finish the same way the rest of the project does, it can look seamless if done correctly and it doesn’t have to stop your progress completely.

Fixes of this sort would apply to a table top with a gap/seam of 1-5 mm (up to ~3/16th of an inch) or a tenon shoulder that doesn’t meet perfectly with its mortise housing (gaps of 1-2 mm only). On a table top, using a trim router or a scratch stock, create a seam that’s deliberately a little larger than the seam gap. Make the width consistent and scratch out a depth of ~ 3 mm (1/8th inch). With the gap now of uniform size, size a piece of scrap wood (make sure to pay attention to grain color and patterns) to the size of the new opening. Make the depth of this filler piece slightly thicker than the depth (4-5 mm) and plane off the excess once it’s glued in place and dry. If you were careful with your color and grain selection, the fix should be virtually invisible.

For tenon shoulders, carefully plane a thin strip of scrap wood to the size of the tenon gap and insert it. Check the fit before you commit to gluing it up. Once you’re satisfied with the fit, glue it in place and use a sharp chisel to pare away the excess once the glue dries. Again, careful selection of wood for color and grain make this type of repair virtually invisible.

Filling with epoxy

The options we’ve discussed so far are all intended to hide seams in woodworking projects. But there is another option which is becoming popular; that of turning those seams into a design element of your project. This has become fairly common for tabletops made out of natural wood slabs with what are known as “live edges.” When those slabs have fissures in them, either due to being cut from a “V” formed by a separating trunk or from cracking in the slab itself, it is common to fill the opening with clear or colored epoxy.

This same technique can be used to fill gaps and knotholes in any woodworking project, turning what would otherwise look like a mistake, into an attractive element in your project’s design.

In order to do this, you need liquid epoxy, the kind that is normally used for fiberglass work, not the kind that is used as an adhesive. While both are epoxy products, the basic difference is that the liquid epoxy is thinner, making it pourable. It is available in bottles from 8 oz. up to a gallon. Depending on the actual product you buy, it may mix at anywhere from a 1:1 ratio up to a 1:4 ratio. I’ve used various different mixtures and don’t see any real difference in the results. However, the viscosity of the epoxy is important.

These epoxies can be used clear, although it is not uncommon to tint them. A wide variety of tint techniques and materials are used, such as dying it blue and making the crack look like a river; but for the sake of this article, we are going to stick with black, the most common color. To make the epoxy black, you can add either artists fine powdered charcoal or black iron oxide (usually imitation iron oxide, which means it is not rust).

The other important thing about your epoxy selection is the cure time. You don’t want to use a “fast dry” epoxy, which has a cure time of five minutes. Rather, you’re better off using one which provides a 15 to 30 minute cure time. Longer is fine; but not necessary.

Preparing and using the epoxy

In order to use the epoxy, you’ll first need to first determine how you are going to pour it into the gap you are trying to fill. If possible, you’ll be better off pouring the epoxy in from behind; but that requires that the gap goes all the way through the workpiece. If it doesn’t you’ll either need to fill the gap from the front or drill a hole through to the gap from behind. While pouring in the epoxy from the front might be easier to accomplish, it may not turn out as smooth. You’re better off drilling that hole from the back.

To make the pour from the back side of the project, first seal off the gap with the same sort of aluminum foil tape used for ductwork. I’m not referring to duct tape or duck tape here, but rather a product made out of aluminum foil. It will provide a smooth surface for the top side of your project. However, the tape doesn’t have a very strong adhesion rate when applied to wood, so you’ll need to press it down well to ensure that it sticks well and the epoxy can’t run out.

Use one continuous piece to cover the entire gap. You can also use other types of tape, such as masking tape for this. The advantage of using the aluminum foil tape is that it will leave a very smooth surface.

With the “mold” prepared, you’ll want to mix up your epoxy, following the ratio marked on the container. I use graduated mixing cups for this, which allow me to get an accurate and consistent ratio. If you are using a pigment, add it in right at the beginning of mixing the epoxy.

Pour the epoxy slowly into the gap, giving it time to work its way down and fill all available space. To help air escape, don’t fill the full length of the gap right from the start. Rather, work from one end to the other, allowing the end you start at to become totally filled, before filling the other end. Once it is filled, you may see some air bubbles form, indicating that you didn’t give enough time for all the air to come out. That’s okay, just make sure that the gap is slightly overfilled, with extra material on the surface. You can chisel and sand that off later.

If you are working from the top side, the process is the same, with the exception that you need to be extra careful to ensure that you overfill the gap, leaving you material that you can sand off. One potential problem with this is that the epoxy will fill the grain in the wood, so if you are planning on using stain on the project, you’ll want to stain the wood, before filling the gap with epoxy.

Filling cracked knotholes is just like filling gaps from the top side of the table, unless you have an open knothole. In that case, you’ll want to work from the back, ensuring that you have a smooth surface on the top of the project.

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