Selecting Healthy and Environmentally Sound Finishes

by GreenHomeGuide Staff, 08/16/05

Alex Pennock wrote this article, with contributions from Marian Keeler and Willem Maas.

Clear finishes protect wood from wear, moisture and aging, but they can degrade the environment—in your home and elsewhere. When selecting a clear finish, we recommend considering the environmental impact of its manufacture and disposal, its effect on indoor air quality, and its durability.

The environmental impact of clear-finish manufacture and disposal ranges from high (for the energy-intensive and polluting manufacture of petroleum-based solvents) to low (for the extraction of natural oil from tung tree nuts). All clear finishes must be disposed of as hazardous waste, so it’s best to buy only the amount you need.

Like paints, clear finishes can contain high levels of volatile organic compounds (VOCs), which contribute to air pollution and can cause health problems. (See About VOCs for more information.) Finishes have a greater effect than paint on your home’s air quality, however, because they must be reapplied more frequently on high-use surfaces like floors, tables and chairs. Toxic and polluting ingredients such as toluene and xylene are common solvents; carcinogens such as arsenic, beryllium, and cadmium can be used as drying agents in linseed oil; and toxic glycol ethers may be used as solvents in many water-based sealers.

All clear finishes are made of a carrier, or solvent, which keeps the finish liquid in the container and evaporates as the finish dries, and a binder, or resin, which protects the surface. Finishes may also contain additives to speed drying or provide fungicidal properties. All these elements contribute to a finish’s environmental impact.

Solvents can be derived from petroleum-based or synthetic materials as well as from materials as simple as linseed oil or water, and can vary within a type of finish: some types of lacquer, for example, use water as a solvent while others use oil. The binder determines a finish’s function, durability and category (lacquer, natural oils, shellac, and so on). Additives may be toxic, though not all are.

Clear finishes vary in their durability and ability to protect wood from heat, water and chemicals. Lacquer, shellac, varnish and water-based sealers coat wood, forming a protective layer on the surface; natural oil sealers penetrate wood, prolonging the wood’s life but leaving the surface susceptible to scuffs and dirt.

HEALTH AND ENVIRONMENTAL PROS AND CONS

For a summary of the environmental pros and cons of the materials discussed here, see GreenHomeGuide’s Buyer's Guide to Clear Finishes.

Lacquer (acrylic or nitrocellulose binder)

The binder in most lacquer is acrylic, which is synthetic, or nitrocellulose, which is derived from natural sources such as wood or cotton. Acrylic lacquer is more durable and better for floor coating, while nitrocellulose lacquer is more often used on furniture and decorations and can be touched up easily because it does not set in the same way as acrylic lacquer. While nitrocellulose comes from natural sources, its benefit is outweighed by the environmental and health impacts from xylene, toluene and solvent manufacture and exposure. Since lacquer does not form as hard a coating as varnish or water-based sealers, the wood is more susceptible to water and physical damage.

Because lacquer has high solvent and VOC content and often contains the solvents xylene and toluene, conventional lacquer is not considered environmentally preferable. In fact, conventional lacquer is one of the worst clear finishes in terms of emitting of VOCs into your home. However, finding environmentally preferable varieties is difficult. While many products fall below the EPA’s VOC limit for lacquer of 350 grams per liter (g/L), water-based lacquers (see the Water-Based Sealer section below) eliminate many toxic ingredients and significantly lower VOCs.

Natural Oils (linseed or tung oil binders)

Natural oil finishes are slightly processed linseed or tung oils derived from renewable resources—flax seeds and tung tree nuts, respectively. They are moderately durable compared to other finishes. Natural oils penetrate wood to protect it from within and so do not protect the surface as well as varnish or lacquer, but still offer moderate water resistance and are environmentally sound because they contain few additives and are less processed than most finishes. They are lower-VOC than lacquer and varnish, despite being mostly oil, because they cure by absorbing oxygen and hardening rather than by evaporating and simply leaving behind a binder.

However, these oils must be thinned with a solvent, which will increase the amount of VOCs released during application. When thinning, use as little thinner as possible to achieve an easily spreadable consistency. "Boiled," "heat treated" or "polymerized" linseed oil contains a metallic drying agent to cut drying time to a day. Raw linseed oil, without drying agents, can take a week to dry, so it is rarely used, though its manufacture is simpler and less polluting since it lacks metals. Tung oil cures fully in a few days so it does not require drying agents, though they will speed up drying times. Tung oil is a bit more water resistant than linseed oil. Few people are allergic or sensitive to these oils.

Wood finished with natural oils is easy to touch up by simply rubbing in another coat every year or so, avoiding having to sand down the floor and reapply when areas start to fade, as often is the case with varnish. Check the Materials Safety Data Sheet (MSDS) for carcinogens arsenic, beryllium, chromium, cadmium, and nickel and the probable carcinogen lead, which may be used as drying agents. Cobalt and zirconium are nontoxic drying agents.

Refer to GreenHomeGuide’s Clear Coatings Directory for Natural Oils reviewed and approved by our Subject Editors.

Shellac (lac resin binder)

Shellac is more environmentally sound than lacquers and varnishes and is suitable for floors that are not subject to much scuffing, heat or moisture, as it is not very durable under these conditions. In high-wear applications shellac should be coated with a water-based sealer because of its low durability if left unsealed.

Shellac’s natural resin binder, produced by the lac insect, is considered a renewable resource, and very few people are sensitive to the resin. Shellac uses ethanol, an alcohol solvent made from corn. During application, vapors from ethanol can irritate your eyes and respiratory system but they dissipate quickly as shellac dries.

Shellac can also be made with methanol, a powerful toxin also known as wood alcohol. This is uncommon, but you should check the MSDS. Buying the correct amount to cover your surface is especially important, since shellac has a shelf life of only six months. Because shellac dries quickly, you can apply many coats in a short time—dust has less time to settle, which makes desirable results easier to obtain.

Varnish (acrylic or urethane binder)

Varnish is typically a mixture of a binder, commonly urethanes or acrylic, and drying oil, usually linseed oil, used on indoor surfaces. Varnishes are the best clear finish for resisting heat, chemicals and solvents, and they are very widely used. Because varnishes use petroleum-derived and synthetic solvents that emit high levels of VOCs during application, varnishes are not considered environmentally preferable, but they are the most durable floor finish.

During application and drying, which can take up to eight hours per coat, varnish emits the highest level of VOCs of the clear finishes discussed here. Achieving the proper finish will require three or four coats, each of which needs to dry and be sanded before the next goes on, exposing you to high VOC concentrations for least two days. Varnish is toxic to handle, and cleaning up requires harsh solvents that release more VOCs into your home. And the high levels of petroleum-derived solvent content mean the manufacture of varnish is environmentally damaging. On the plus side, once varnish has dried, its impact on air quality is only moderate. Varnishes are the longest-lasting finish, so you will not have to reapply them as often, reducing your exposure to their high VOC concentrations.

The EPA has set VOC limits for varnishes at 450 g/L, though some states have recently stipulated that 350 g/L is the upper limit for all architectural coatings. VOC levels are listed on a varnish’s MSDS or Technical Data Sheet and are rarely much lower than required by law. For comparison, water-based sealers can easily be lower than 200 g/L, down to 50 g/L. Urethanes are more expensive but more durable than acrylic varnishes.

Water-Based Sealer (acrylic or urethane binder)

Water-based sealers are environmentally preferable, and they are still being improved. We recommend them for their low environmental and health impacts, ease of handling and clean up, and durability. Most low-VOC clear finishes use primarily water as a solvent, and acrylic and urethane as binders (the same as lacquer and varnish). Not all of the solvent is water, however. Glycol ethers, which are toxic, are still often used as solvents in small amounts.

Because water-based sealers use fewer harsh solvents, petroleum derivatives or other toxic substances, they avoid the environmental damage and pollution associated with the manufacture of these ingredients. Clean up requires water instead of harsh solvents that affect indoor air quality. Water-based sealers also dry very quickly, allowing you to apply many coats in a day and reducing the time you are exposed to heightened VOC levels, but they also require quick and more careful application.

When shopping for water-based sealers, a low-VOC label indicates 350 g/L or less. Many products achieve levels as low as 50 g/L and will display this information on the label or have it on their MSDS. The MSDS will also tell you if the sealer contains glycol ethers. If you must use a sealer with glycol ether, propylene glycol and ethylene glycol are less toxic types. Water-based sealers offer moderate water, heat and solvent protection; while they are less durable than oil-based varnishes they are more durable than traditional lacquers and other finishes discussed here.

According to Mark Knaebe of the U.S. Department of Agriculture Forest Products Laboratory, catalyzed urethane acrylics are the toughest water-based indoor floor finishes regularly used. "Catalyzed" means the sealer cures by chemical reactions rather than simply water evaporation; because of this, they are difficult to apply but provide very good protection.

Refer to GreenHomeGuide’s Clear Coatings Directory for recommended water-based sealers.

RESOURCES

GreenHomeGuide’s Clear Coatings Directory
Our list of clear finishes approved by our panel of subject editors, based on their research and experience with the products.

Green Seal's Wood Finishes Report
A report by Green Seal, an independent, nonprofit certification organization, providing background and selection criteria for clear finishes and stains.

More Background Information on Wood Finish Types
This article by Floor Covering Installer Magazine describes durability, application and aesthetics of many of the wood finishes discussed above.

Everything You Need To Know About Finishes
Canadian Home Workshop’s guide to clear finishes, focusing on application.

United Way's How to Calculate Material Needs
Guidelines for estimating the average number of square feet of surface a gallon of different coatings will cover.