Most people who repair minor damage to finished wood are familiar with resin burn-in sticks, polyester fillers such as Bondo and epoxies of various sorts. Far and away the most versatile filler, almost a perfect replacement for wood, is the epoxy stick generically called “Tootsie Roll” epoxy.
Epoxies have been used for years as fillers for gouges, dents and other unwanted cavities — sometimes in the usual form of resin and hardener, often with a bulking material of some sort added.
For a respectable period of time, the most versatile type of epoxy used for filling comes in the form of a stick or cylinder about 7” long and 3/4” in diameter that contains a center core of hardener and an outer sheath of resin. Because of its shape and two-part construction, it’s commonly referred to as “Tootsie Roll” epoxy.
Mixed within both parts of the stick are micro-balloons — microscopic, hollow-glass spheres that add stiffness to the epoxy, allowing it to be formed or molded without sagging or flowing and adding to its machinability. This form of epoxy has characteristics that resemble wood more than any repair material other than wood itself.
A formidable tool
Epoxy in this form is remarkable stuff. It’s strong enough to hold screws. It can be molded like clay and carved like basswood. It won’t shrink, swell or gloss up under lacquer or heat, which means it can be molded or tooled right to its final shape. It can be applied before finishing and can be put through a drying oven without softening. It can also be machined, sanded and, because of the micro-balloons, will take stain reasonably well. It is strong enough to be used for structural repairs, has virtually no shrinkage and is dimensionally stable over a wide range of temperature and humidity.
In addition, the epoxy will bond to most clean surfaces, including metal, glass and, of course, wood. It won’t stick to Mylar or polyethylene-type plastics, however, which makes these materials ideal for use as release paper (or cauls) when duplicating carvings and moldings.
If there’s a downside, filling with epoxy sticks usually takes a little longer than filling with resin burn-in sticks because the epoxy takes longer to cure. Also, epoxy sticks aren’t available in clears, which are very effective for filling dents where no additional color is needed.
There are other forms of epoxy, but stick epoxy is much easier: slice off a section, knead until it becomes a uniform color and apply to the damaged area. There’s no messy mixing required.
Most epoxy sticks come in a neutral color, but some companies make them available in various wood tones, which can be intermixed to create an extensive palette. You can also make your own color to match a specific background by adding colorants that mix with lacquer. These include touch-up powders, universal colors, base concentrate or Huls (now Chroma-Chem) 844. But keep in mind that adding too much colorant can affect the epoxy’s curing properties. To make the color mixing with the powders go easier, wet the powders or the epoxy stick with water.
Wetting your fingers and tools will also help you shape and mold mixed epoxy and the heat from a hot burn-in knife will make the epoxy softer and easier to smooth. The heat also helps keep the epoxy from sticking to the blade.
On the job
The working life of epoxy sticks is measured from the time the epoxy reaches a single color during mixing to the time it can no longer be shaped easily by hand. For standard-cure sticks, this is 8 to 10 minutes; for fast-cure epoxy sticks, it’s 3 to 5 minutes — both at 75 F.
Two key points to remember: Large masses of epoxy cure faster than smaller ones and heat speeds the curing process while cold retards it.
Hand-shaping an epoxy fill
Select an epoxy stick to approximate the background color you want for the missing wood. Slice off a section large enough to give you enough mass to fill the void. If the amount you need is less than a thin slice, divide the slice into halves or quarters. A color-mixing guide, available from most manufacturers, can help you create a more accurate color match.
Cleaning up safely
Some people can become sensitized to the chemicals in epoxy and should wear gloves when working with it.
For cleaning up uncured epoxy, isopropyl alcohol and lacquer thinner can be used. But these solvents should not be used to clean epoxy from your skin because they can carry the chemicals through the skin and into your body. You should use soap and water to clean your hands.
In addition to lacquer thinner and isopropyl (rubbing) alcohol, acetone acts as a mild solvent for uncured epoxy, with isopropyl alcohol having the least potential for damaging a finish.
Knead the epoxy thoroughly until it becomes a single color. If you don’t have access to pre-colored epoxy sticks, mix in the appropriate color during the kneading.
Apply the epoxy to the void, shaping and smoothing it with your fingers or a tool moistened with water. When the epoxy has hardened, shape and smooth with sandpaper, a chisel or a carving tool. Next, add graining and more color, if necessary, and apply a topcoat finish in the same way you would to a burn-in.
Shaping with a hot burn-in knife
You can use a hot burn-in knife without a lubricant to shape an epoxy fill until it cures, but care must be used because while the hot knife softens the stick and makes it spread and smooth more easily, the heat also speeds up the curing and the epoxy can get too hard.
For a short time after the epoxy has become too hard to manipulate with the hot knife, it can still be sliced with a sharp tool like a chisel (or even the sharp edge of a burn-in knife of the type used with a burn-in oven) heated to 350 to 450 F. This technique is limited to unfinished wood, plastic laminates and other materials that aren’t affected by the high heat.
Leveling with a burn-in knife
Leveling a fill on a flat surface using an epoxy stick and a burn-in knife is almost identical to the method used to level a burn-in stick. First, press the epoxy firmly into the void so it slightly overfills and apply burn-in balm, Vaseline or Wool-Lube (gel soap) over and around the fill. These products act as a release agent.
Leveling using a Mylar strip
An epoxy fill can be leveled easily using a Mylar strip and a credit card instead of a burn-in knife. First, cut a strip of clear Mylar so that one end is shaped as closely as possible to three sides of the void and attach a piece of masking tape to the other end. Fasten the tape to the surface so the shaped end of the Mylar just covers the void. Fold the Mylar back so it is out of the way and press mixed epoxy firmly into the void. Lay the Mylar back over the fill and squeegee out the excess using a credit card or similar object. Remove the excess with isopropyl alcohol before the epoxy hardens.
This article originally appeared in the October 2013 issue.