Whether you are working on siding, furniture, or interior trim, a nice finish coat can make a lot of difference. And what you use as a finish coat makes a difference, too. For exterior finishes, the primary characteristics we usually look for are durability and ease of application. For interior finishes, though, we have to be a lot more careful because we will be breathing in chemicals from some of those products as long as we live in our home.
According to The Whole Building Design Guide (www.wbdg.org), a program of the U.S. National Institute of Building Sciences, there are two kinds of finishes, surface and penetrating.
Surface finishes cure hard, can be built up in layers, and include shellac, alkyd and polyurethane varnishes, lacquer, water-based and latex-based semitransparent stains, and solid-colour stains. Penetrating finishes are oil-based and don’t cure to a hard film. These include oil finishes, such as tung and linseed oil, and oil-based stains.
Generally speaking, the surface finishes contain more chemicals. For centuries, varnish (a catch-all term for clear wood finishes) was made from resins collected from natural products such as tree saps or insect secretions and mixed with linseed or other natural oils. To create the final product, the mixture was thinned with turpentine. More recently, synthetic resins derived from coal tar and petroleum began to replace the natural resins, and petroleum distillates became the most commonly used solvents. The petroleum-based products in the synthetic resins helped increase durability
But, beginning in the 1970s, there were increased concerns over the impact of surface finish chemicals on human health and the environment.
Resins and Solvents and Driers: Oh My!
Pigments or dyes add colour and hide flaws. Resins, or binders, are the natural or synthetic film-forming component, and can include acrylics, vinyls, alkyds, cellulosics, epoxies, polyurethanes, and oils. The particular type of resin determines the finish’s hardness, flexibility, and resistance to stains, solvents, and water. However, resins are hard and dry, and you need them to be liquid to apply them.
This is why solvents and/or thinners are added to liquefy the resins so you can brush them on. Solvent-based coatings typically use organic solvents such as alcohols, ketones, glycol ethers, petroleum distillates (mineral spirits, kerosene, toluene, xylene, benzene and naphtha), and turpentine. These products, particularly benzene, toluene, and xylene have toxic effects, primarily on the respiratory system.
You also need a chemical to kick the solvent out of the liquid quickly so your finish isn’t sticky for days. So, cue the driers. A drier is a substance that accelerates the drying of oil-based paints, varnishes and inks. Metallic salts of manganese, cerium, lead, chromium, iron, and zinc, are common and effective driers. These heavy metal compounds are toxic when ingested or inhaled. There are clear health risks for those who manufacture them; in use they are considered a health risk only in long-term exposure, but the studies that accurately test long-term exposure in homes are few and far between.
What are Volatile Organic Compounds?
O.K. When I lose my temper, my wife claims I am a volatile organic compound but, according to Health Canada, volatile organic compounds (VOCs) have boiling points roughly in the range of 50 to 250 °C. This includes the solvents and driers in some finishes and, as they evaporate during the drying/curing process, VOCs are released into the air, affecting indoor and outdoor air quality.
The American Lung Association reports that VOCs and their byproducts can produce a number of physical problems, including eye and skin irritation, lung and breathing problems, headaches, nausea, muscle weakness, and liver and kidney damage. VOC levels can be 10 times higher indoors than outdoors, with numbers rising up to 1,000 times higher immediately following application of a new coat of finish.
Outside, VOCs released into the atmosphere can combine with each other, or with other substances in the air, to create new chemical compounds, such as ground-level ozone. Ozone is a major component of smog, which causes negative health and environmental impacts when present in high concentrations at ground level. VOCs are considered air pollutants, and the amount that can be released for a given amount of solids is now regulated in many areas.
It may not be a good idea to automatically reach for products containing volatile organic compounds (VOCs). There are so many products that use glues and finishes containing VOCs now that you really have to read the label.
A second test for VOCs is “if it smells strong or smells like a chemical, it probably is.” A third test is whether the label suggests a respirator, long-handled tongs, and a hazmat suit during application.hazmat suit
So what can you use?
There are many alternatives: low VOC finishes, milk and whey-based finishes, natural finishes like shellac and spar varnish, and penetrating oils like linseed oil, which we used on all the B.C. fir beams in our underground home.
I will finish with a discussion of these finishes next time.