DEALING WITH DEFECTS
by J Dewitt
In my cabinet-making and finishing business, we always allow 20% stock overage in our quotes for furniture. This is to accommodate various defective areas in the rough stock that most customers object to: knots, internal checks and splits, worm holes and sapwood. While it can be argued among woodworkers that these areas are what gives wood a special type of beauty, the public usually regards them as defects, so I err on the side of caution and try to design around them.
The last commissioned piece I built was a cherry settle and my wood supplier said that he had approximately 70 bd. ft. of cherry that he would give me a discount on since it had more that the usual amount of sapwood, knots and end checks. After extracting the clear stock for my customer,
I figured there was just enough stock left to build a smaller version for myself. The only problem was how to deal with all the splits, knots, sapwood and worm holes. In this article I'll show you how I turned 30 bd. ft. of stock normally destined for the scrap box into a cherry settle.
Before starting with the various techniques I use, I would like to impress upon readers the value of having a clear, detailed scale drawing of the piece you are building as well as a detailed cutting list. This will help you to visualize how the different parts of the project relate to each other and which parts show the most. A detailed drawing is also essential for the first method of dealing with defects, which is to simply design around them.
Designing Defects Out
Probably the easiest way of handling defects is to incorporate them into a part of the design so that they are hidden. This is what I did with the knots on the back of the settle. Many knots that seem large on one side disappear almost entirely on the other side of a 3/4" board, so putting defects on a back or non-show side works nicely. However, be sure to take into account that furniture gets moved around frequently, so what may be a side facing a wall one year may be a show side later on. The large knots on the oak legs in the photo of another commissioned piece were dealt with in a similar manner, I designed them so that they faced to the inside of the side table. Once the piece is completed with the drawer, they will never be noticed. Because this weak area is near the location of a mortise, I filled the large knot hole with a polyester filler (like Bondo) to give it strength, then touched it up with pigments and shellac. (This will be discussed later.)
Another way to handle large defective areas is to place them in the initial layout so that they fall on an area that will be cut out. This is what I did with a large worm-holed area on the right side of the settle. Most of the holes fell in the area that I cut out for the arm. A few holes were left, but they were filled and touched up later.
For the back panel below the seat, I saved all the sapwood cut offs and small width pieces and glued them up to make a large 8" wide piece. This panel is never seen when the seat is down so I wasn't really worried about the appearance of this piece. However, because it is a structural part that supports the weight of the person sitting in the settle, I avoided wood with splits, checks, and large knots. (This is an important point to remember, any part that is a load-bearing piece should be as defect free as possible. Sapwood does not affect structural integrity.)
Dealing With Sapwood
I don't always regard sapwood as objectionable, rather, I really like the sharp contrast between light and dark, especially on dark woods like walnut and rosewood. But you have to be careful that it doesn't interrupt the natural flow and form of the piece, like the sapwood on the front panel of the settle. I didn't notice this when I was building the piece, because in freshly cut cherry, the distinction between sapwood and heartwood is not that obvious. After a month however, the contrast became very obvious when the heartwood darkened. It's wise to wait anyway before correcting this problem, so that you can match the sapwood to the color of the heartwood after it's darkened.
I handled this problem by darkening the sapwood using alcohol soluble dyes. Using a fine pointed red-sable artist's brush, I mixed several dye colors together until I got the sapwood to blend in with the heartwood. I find that working with several layers of slightly different thinned colors gives a much better final color match than trying to hit the color all in one shot. Once I padded on several more coats of shellac, the sapwood was hardly noticeable.
This technique can also be used on pieces that are unfinished. Here I use water-soluble dyes, because they are easier to handle and do not dry so fast. I find spraying the dye with a small touch-up gun works best. The trick with this technique is to anticipate what the wood will look like when it's finished, so I find wetting down the wood with water first will give a close approximation of what the wood will look like finished. This technique also works if you're planning on staining the wood with a pigmented wiping stain like Minwax. The sapwood will literally disappear after staining.
I really like knots, and particularly, the area around knots. As mentioned above, areas that will be subjected to stress should be free of knots but other areas, like the upper right top of the settle are open ground for incorporating knots. The area around a knot usually has very swirly dense grain, so care should be taken when finishing, and the use of hand scrapers is suggested. Knots that have large cracks or are loose need to be consolidated so that they do not fall out or chip later. I use several techniques for this depending on how bad the knot is.
Where large cracks have developed, or where large pieces of the knot are missing, small pieces of wood should be glued in with epoxy tinted with pigments. This is especially true of knots in walnut. Mix some dry pigment powders into five minute epoxy and glue in small pieces of wood cut into small enough pieces to jam in the cracks. (Tip -- rather than mixing the epoxy in the suggested 1-1 ratio, I mixed the epoxy in a 1.5 resin to 1 hardener. This results in a much harder glue that scrapes better when fully cured). When completely cured (24 hours) cut down the wood slivers with a flush cutting saw. Notice my homemade one in the photo. It's nothing more that an old hacksaw blade with the set banged out on an anvil. I wrap tape around one end to act as a handle. Then I finish the surface with a hand scraper.
With smaller cracks, or just to consolidate a loose knot, cyanoacrylate glue can be used. I use the water-thin viscosity because it actually wicks into the cracks by capillary action, but I usually put tape on the other side to keep the glue from seeping through. If you use the accelerator sold with these glues, the area can be scraped and sanded immediately. Don't sand before the glue is completely cured, because the sawdust will mix with glue and form a light filler that doesn't look appropriate with the dark knot
With large knots that are missing or are powdery and brittle, I usually scrape out the loose pieces and fill the area with a catalyzed polyester filler, such as those sold in auto-body stores. If you buy these type ask for white hardener since red hardener is too strong a color when dry. These fillers also dry quickly, and I sand the area flush after about an hour. They also have the advantage of not shrinking, which is what will happen with water-based fillers. When dry, I rub some pigmented wax sticks over the area to fill in the small imperfection and bubbles on the surface of the filler and then blend the area into the rest of the wood area using shellac and dry dyes and pigments. As mentioned above I try to hide these large knots as much as possible in the design.
Cracks and Splits
During its life, trees may be subjected to growing stresses which cause internal cracks and splits in the structure of the wood. This is known as reaction wood. This is different from end checks and cracks at the end of the board which form as the board dries. These should always be cut out. On the other hand, cracks and splits in the center of an otherwise good board can be filled with dark tinted glue which has a nice aesthetic look. Very small cracks can be filled with water-thin super glue like "Hot Stuff" sold by Satellite City. Simply hold the tip of the dispenser against the and the glue will wick into the crack. Tape the other side of the board if the crack goes all the way through. When the glue dries it will look just like a grain line and blend in perfectly. If you don't want it to appear dark, start sanding with 100 grit sandpaper before the glues dries. Doing this kicks up some sawdust and push it into the crack along with the glue and dry to almost the same color as the wood.
For large cracks, I use the same technique as the epoxy technique describe above for knots. Using the dry colored pigments, you can tint the epoxy light or dark to create the effect that you want. Several applications may be in order, because the epoxy will shrink slightly as it dries. The gap-filling advantage of epoxy is that it also strengthens the weakness in the board caused by the crack. This is what I did on the left side of the settle, where long cracks went right into the wedged through tenons. The tenons were a tight fit and the glue-filled crack stayed put when I drove the tenon home during glue-up.
I can't say that I like worm holes so I usually like to place them on areas that will be hidden or cut out entirely. The other disadvantage is that they severely weaken the wood, so care should be used when using boards that are badly worm-ridden. Several techniques can be used, but I like to fill them with tinted epoxy and then touch them up with shellac and dry powders to blend them into the rest of the piece. You can also use a water-based filler and then tint them with the dry colors. They dry lighter than the color when wet, so you may need to experiment a bit to get the feel of this technique. The filler I made dried too dark, so I had to lighten the dried filler up with shellac mixed with dry pigments. Another quick and easy technique for worm holes are pigmented wax sticks sold by several suppliers like Behlen or Mohawk. Simply push the wax into the hole and then scrape off the excess with a wooden chisel like in the photo. Then smooth it level with the paper back of some lightweight sandpaper. Contrary to what you might think, most finishes can be applied over the wax, as long as the area isn't too large.
The extra time I put into fixing up the left over stock for the settle was worth it. I figured that I saved over 100.00. That's a good enough reason for me to try and save it from the kindling bin. It may take a little extra work, but my wallet sure felt the better for it.