The next time you’re preparing to finish a woodworking project, take an eco-moment to consider the environmental friendliness of the finishing materials you use. Conventional solvent-based finishes such as nitrocellulose lacquer and varnishes provide a durable, high quality finish at a reasonable cost. However, they can also be significant sources of volatile organic compounds (VOCs) and hazardous air pollutants (HAPs) both in the application and cleanup stages. Solvents released to the air include xylene, toluene, ketones, methanol, methylene chloride, and various mineral spirits.
There are application techniques to minimize the harmful environmental effects of solvent-based finishes but these add cost and complexity to the finishing process. An alternative approach is to use non solvent-based finishes or finishes with low solvent content. This category includes finishes made from plant oils and waxes, insect secretions, earth and mineral pigments, and other natural materials. It also includes various types of water-based finishes.
Oil finishes are derived from seeds, nuts, and other plant parts. These are penetrating finishes that seep into the wood pores rather than building a protective film on the surface of the wood. Linseed oil and tung oil are the most commonly used oil finishes. Unlike most other oils, these two oils cure – they change from a liquid to a solid by absorbing oxygen from the air (a process called polymerization). As a result, they work fairly well as finishes (more details below). Oils such as mineral oil and olive oil do not cure or solidify, so that they are ineffective as finishes. In between are the semi-curing oils such as walnut oil, soybean oil, and safflower oil. They cure very slowly and never very hard. They are only marginally effective as finishes.
Linseed oil is an oil extracted from the seeds of the flax plant. Because raw linseed oil does not fully dry, most woodworkers use “boiled” linseed oil in which the oil has been oxygenated to allow it to dry and harden. Varnish or driers may also be added to speed up the drying process. Unfortunately, such additives can compromise the environmentally friendliness of the finish. Linseed oil provides minimal scratch and stain resistance and is easily penetrated by water and water vapor.
Tung Oil is a natural finish that is made from the nuts of the Tung tree. It has been used for centuries by the Chinese to protect furniture and other wooden items. Compared to linseed oil, tung oil cures without the need for drying additives, and it does not darken the wood as much. It also provides better scratch resistance and water resistance as long as five or more coats are applied. Tung oil tends to cure slowly and can turn a whitish color if applied too heavily.
As a group, oil finishes are the least protective of all finishes except for wax. Although they are not the best choice for table tops, floors, and other high traffic applications, oil finishes do have their advantages. A hand-rubbed oil finish enhances the natural color and warmth of wood in ways that other finishes cannot ( although a periodic “refreshening” is generally required.) Oil finishes are easy to apply and to repair. Light scratches and scuffs can be repaired by simply recoating the affected area with more oil. Another advantage is that they are flexible and move with the wood. They are also food-safe so they are ideal finishes for cutting boards, wooden salad bowls and wooden utensils.
Shellac is made from a secretion produced by the lac bug, a tiny scale insect that feeds on trees in India and Thailand. The lac resin is processed into flakes which are then dissolved in alcohol to make a finish that can be brushed or sprayed onto wood surfaces. The colors of shellac range from nearly transparent to a natural amber to reddish-brown. Shellac is one of the oldest and perhaps most underrated of all finishes. It was once considered a premiere finish for fine furniture but, like other natural finishes, it has fallen out of favor in modern times with the introduction of synthetic solvent-based materials. Compared to these newer finishes, shellac provides limited resistance to water, alcohol, and heat. If you’re looking for a durable finish for a table or counter top, shellac would not be at the top of most people’s list.
However, shellac does have a number of positive attributes. It is quick drying and doesn’t darken with age. like varnish finishes. It provides a good barrier against water vapor and stains. It is an excellent touch-up material for other finishes. It works extremely well as a sealer and as a barrier coat to contain contaminants such as silicone, dirt and grease. It is one of the few finishes that both adheres well to other finishes and that other finishes adhere to. And, shellac is a perfect finish for items that come into contact with food or children’s toys because the resin is edible. In fact, it is even used as a coating on candy and pills.
Nothing beats a good shellacking!
Water-based finishes have become increasingly popular with eco-minded woodworkers in recent years because they contain significantly less solvent than conventional solvent-based finishes. This means less solvent to evaporate into the atmosphere and less solvent to breath. Healthier for the environment and healthier for your lungs. That is the value proposition of water-based finishes.
Water-based clear finishes are typically milky white in color as a liquid but turn clear as they dry. They are clearer than their solvent-based counterparts and stay that way over time. This makes them ideal finishes for light-colored woods such as ash, birch, and maple. They also dry quickly. In less than an hour, you can apply two coats of finish with a sanding in between. Of course, this quick drying means you must work quickly and avoid excessive brushing when applying the finish. Clean up doesn’t require any smelly chemicals – just plain old soap and water. That’s another plus of water-based finishes.
In terms of toughness and scratch resistance, water-based finishes score very highly and compare favorably with solvent-based polyurethane. Their durability, in combination with reduced fumes, has made them become increasingly popular as a finish for hardwood floors. Note that once a water-based finish has cured, it is more difficult to remove than a solvent based finish. Conventional varnish and paint removers hardly work at all with some urethane finishes.
The marketing terminology of water-based finishes can be a bit confusing. You’ll see them sold as “lacquers”, “varnishes”, “urethanes”, and “polyurethanes”, implying that these finishes are similar to solvent-based finishes of the same name. The reality is that water-based finishes have more in common with each other than with solvent-based finishes, regardless of the name. Both classes of finishes do contain many of the same ingredients, such as acrylic, and polyurethane, but the resin transport and curing mechanisms are quite different.
There are a few drawbacks to water-based finishes. They are pricier than solvent-based finishes. They are less resistant to heat, solvents, and acids than solvent-based finishes. And because they contain water, water-based finishes have a tendency to raise the grain in wood. However, this only happens with the first coat and there are ways to minimize it. One is to use a fast drying, spray type water-based lacquer. Another is to use an alcohol-based sealer such as shellac before applying the finish.
If you’re looking for an antique-looking colored finish that is biodegradable, non-toxic, and odor-free when dry, then milk paint fits the bill. It is an earth friendly finish that is made from milk casein, clay fillers, lime, and earth pigments for color. Although milk paint is not a very common finish today, it has proven itself over time and is perhaps the oldest form of paint known. In fact, traces of milk paint have been found in King Tutankhamen’s tomb.
The flat, grainy look of a milk paint finish is quite distinctive. It is a popular finish for reproducing period furniture. Artists and craftsmen also use it to create unique decorative effects. Milk paint dries quickly (no, it does not smell like sour milk), it can be applied directly to sanded wood without the need for a primer or sealer, and forms a hard matte surface when dry. It bonds well with raw wood thanks to a naturally forming resin called calcium caseinate and almost any finish can be used as a topcoat over milk paint. Note that the lime in milk paint acts as an insecticide and fungicide, protecting wood from insect and fungal infestations.