Coffee Grounds and Epoxy
as a filler
I often work with wood that has cracks, critter holes, and other voids that appear for a variety of reasons. Sometimes I will leave the imperfections as they occur, to give character to the wood. In most cases, however, I fill them in, especially if they have rough or jagged edges, or if I just don’t like so much “character” in the piece.
The material I use most often as a filler is epoxy mixed with coffee grounds. It is easy to handle, cures fairly quickly, sands well, turns easily, and takes a finish nicely. Further, the granules of coffee give an appearance that I like.
I realize that opinions differ widely on the various issues being brought forth here. Some will argue that it is a waste of time to deal with such wood in the first place. Some prefer more exotic fillers such as crushed tourquoise, brass filings, or a metal powder. Others may use wood dust bonded with thin CA glue. Many options exist. The great thing is that it is your choice and mine to work crappy wood, or not, and to use whatever filler we find to our liking, or none at all.
The epoxy. I buy a good grade of epoxy from a local hobby shop that caters to the model-airplane crowd. Avoid the stuff sold at the local hardware store and home centers. All epoxies are not the same. Some will not harden properly, and others come in double-syringe dispensers that, for me, are impossible to use.
I use the five-minute variety. But when the coffee grounds are mixed into it, it begins to set up and becomes unworkable after only a couple of minutes. Sometimes, but rarely, I will use 30-minute epoxy if I have a situation where the extra working time is needed.
If a vessel has voids all over, do not attempt to fill all of them with one big batch of filler. It is better to use several smaller batches that set up quickly because holding the piece without getting into the uncured and sticky filler becomes an issue with the 30-minute variety.
The coffee. I take coffee right out of the can that holds the supply for my drip-type coffee maker. However, coffee grounds that have been processed can be salvaged and used if they are dried completely. These yield a finer texture and the grain in the filler is less distinct. The processed grounds also give a mix that is easier to force into a small crack.
Preparing the workpiece. At first, let’s assume we have a bowl turned from sound wood except for a couple of worm holes on its outside surface that we plan to fill. My preference is to go ahead and sand the outside to completion while it is still on the lathe. Further, I will brush on a coat of lacquer sanding sealer and let it dry. The sealer ensures that no stain or discoloration will occur next to the holes to be filled. It doesn’t seem to matter if a bit of sealer runs into the holes.
Typically, I then remove the piece from the lathe. but not before considering the steps I will use to smooth the surface once the filler has been applied. Because the filler turns nicely, you can level the surface with turning tools instead of sanding. However, removing a piece from the chuck and then reinstalling it later often results in enough runout so that further turning creates problems.
If only a few small voids are to be filled, I will commit to doing the sanding off the lathe and remove the piece from the chuck. But if the voids are extensive and I plan to level the surface by turning, I will leave the piece in the chuck. I may remove the piece from the lathe, chuck and all, or leave it right where it is and fill the voids at the lathe.
Mixing the epoxy. What I do, first, is to lay a bead of resin onto the surface of a plastic lid from a can of mixed nuts or from a butter cup, whatever is handy. Then I lay down a similar bead of hardener right beside the resin. For a small job, the beads may be only an inch long. For a larger quantity I may use several beads of resin and hardener, but I keep them separate until I begin the mixing process.
Once the beads are on the plastic lid, I use a thin strip of wood to mix the resin and hardener together. I work quickly at this point but make sure that the two are mixed well.
Adding the coffee. I pour the coffee grounds onto the puddle and then use the mixing stick to press the coffee down into the epoxy. Then I work the mix until all the coffee is wetted with epoxy. If the mix appears thin and syrupy, I add more coffee until I finally get a mix that is fairly stiff and granular-looking. As long as the epoxy wets all the coffee, it will be OK; getting it too stiff is not really a problem.
Filling the void. You must work quickly. Press the filler into the void. Apply enough so that it stands proud of the surface. I use a metal spatula for this but the back of a small spoon works just as well. A spoon with a bent handle is good for working on the inside curved surface of a bowl.
After just a couple of minutes, the mix will begin to set up. First, it will become noticably stiffer, then stringy and hard to spread. At this point, the show is over; that batch is done.
Clean the utensils. I keep a bottle of denatured alcohol close by for cleaning the plastic lid, the mixing stick, and the spatula (or spoon). When cleanup time arrives, I squirt a teaspoonfull or so onto the lid, into what mix remains, and then take the mixing stick and rub over the surface of the lid.
The alcohol takes the sticky out of the epoxy. I then rub the spatula into the alcohol, and finally wipe everything with a small swatch of a paper towel. This whole process takes less than 30 seconds and I wind up with a clean mixing stick, a clean spatula, and a clean lid – most of the time.
Let the epoxy cure. The epoxy will go through a series of steps as it cures, starting off with sticky and still somewhat plyable. Three or four minutes later, it will be not-sticky to the touch and will have firmed up so that it will no longer flow or ooze under its own weight. From eight to ten minutes more it will be rubbery. If you press the surface with the point of a nail, the surface will deform slightly but will spring back.
After about 20 minutes total, the epoxy can be carved (or turned). I sometimes use a sharp carving tool to remove excess material. This simply reduces the amount of sanding required (and reduces the amount of dust produced by the sanding).
Generally speaking, I wait at least two hours before I attempt to sand the filled area. A delay of four hours is better; and overnight is better still. The filler is less likely to clog the sandpaper if it has had several hours to cure.
Sanding. Nothing special here. Sand the filled area until it is flush with the surrounding wood. Go through the grits as you normally do. I almost always use a sanding pad mounted in the chuck of a drill (power sanding) and concentrate on the filled area. For small voids, I find 1” disks held by a Dremel tool to be quite effective.
If the filler material stands way proud of the surface, I might begin sanding with 100-grit abrasive. If the buildup is less, I will probably begin with 150 grit and sand through to 400. In both cases, I try to avoid sanding the wood adjacent to the filler.
A different sequence. If a void or crack is large and located so that it weakens a section of the piece, it might be a good idea to fill the void before doing the final turning. My suggestion is to turn away as much waste wood as you feel you can do safely and then fill the void.
Keep in mind that if you remove the piece from the chuck to do the filling, the piece may not run perfectly true when you put it back in the chuck. Leave enough thickness to be turned away so that a small amount of runout will not be a problem.
A built-up fill. If a large hole goes all the way through the side of a bowl, I will often cut strips of wood to wedge into the hole to form a foundation for the epoxy. Balsa works fine for this and is easy to cut and shape. Once a piece is wedged into the hole, a drop of thin CA will lock it in place. One big advantage of doing the buildup is that it makes the epoxy filler easier to apply, and the process goes fairly quickly.
If gaps remain after wedging the balsa in place, you can pack fine sawdust into the gaps and then bond it together with thin CA.
Be careful not to get the underlayment of wood too close to the surface. Leave room for at least a 1/8-inch thickness of the epoxy filler.
Because the hole is nearly circular, I was able to bridge the entire hole with one piece of balsa, which usually is not possible. It is surprisingly strong. On the inside, I tacked in two additional pieces of balsa and then packed in fine sawdust to fill part of the volume. The surface of the balsa and sawdust filler is at least 1/8” below the contour of the bowl even though that is hard to see in the photo.
After the built-up part is complete, apply the epoxy mixture and let it cure. Then sand it smooth and apply the finish.
Painter’s tape. If a void appears on the rim of a bowl, a strip of painter’s tape can be used to help contain the epoxy until it sets up. A bit of trial and error will soon indicate the best method for doing this. Just be sure the epoxy fills the void and stands slightly proud of the contour of the surface. The tape can be removed easily once the epoxy has cured.
Creating a Decorative Ring
You can take this one step farther and create a ring for decorative purposes. The process is simple; cut a groove, pack it with filler, let it cure, then finish the surface.
Make sure the surface intended for the ring runs perfectly true before you try to cut the groove. True it up if necessary. This ensures the groove will have a uniform depth all the way around the ring and that the finished ring will have a uniform width.
A parting tool is used to cut the groove because the grooves are usually too narrow for a gouge. However, most parting tools do not produce sharp, crisp edges which are needed for the ring to look good. This issue can be avoided by using the long point of a skew to incise a sharp line at each edge, then use the parting tool to remove the wood between the lines.
Cut the groove to a depth of about 1/8”. Some turners recommend undercutting the sides so that the filler assumes the shape of a dovetail and is locked in. The undercutting can be done easily by swinging the handle of the parting tool to the side by about 15º and then advancing it into the cut.
Once the groove is cut, apply the filler, let it cure, then smooth the surface. Small voids in the ring can be filled by pressing in additional filler.
You are not limited to coffee grounds for use as a filler. I’ve used metal powders with epoxy, and a lot of turners use turquoise and other soft minerals. But that is another story.