Wood Finishes
- tannerjanesky
- 6 days ago
- 7 min read
A Guide to Finishes for Renewable Materials and their Durability, Health, and Sustainability
Recently I've been exploring using natural biomaterials to make products to replace their plastic counterparts. Materials such as solid wood, hemp, flax, rice hull, and other biomaterials and composites can serve as renewable building materials for large structures, interior design, and consumer products. On their own, these materials are susceptible to damage from moisture, UV, and wear and tear, so finishing them with coatings not only enhances their appearance but also their durability and longevity. If these products look better and last longer, people are more likely to use them instead of plastics.
As I was researching which wood finishes to use for some products I'm working on, it became evident that choosing one would not be easy. Stains, polyurethanes, waxes, varnishes, oils, shellacs, epoxies—which to choose? If civilization is going to use renewable materials, such as wood, we're going to need to demystify finishes and understand their durability, sustainability, and health impacts.
Different use cases call for different finishes. It's not just a matter of which finish looks the best but which one to use in a specific situation to make the wood last and not harm people or the environment. Understanding the differences in properties, origin, toxicity, and durability helps us make better choices for both woodworking and construction and sustainability.
Bonding Types
Some finishes sink into the wood, reinforcing it from within, while others coat the surface and act as a protective shell.
Penetrating finishes, like tung oil, linseed oil, and cedar oil, are absorbed into the wood’s fibers. Because they seep into the material rather than forming a film, they maintain the wood’s natural texture and flexibility. These finishes are great for those who want a natural look while still offering moderate protection. However, they do require periodic maintenance, as they gradually wear away and need reapplication.
Surface finishes, on the other hand, create a harder, protective layer on top of the wood. Polyurethane, varnish, shellac, and epoxy all fall into this category. These finishes can provide better durability and moisture resistance. However, because they sit on the surface rather than integrating into the wood, they can crack or peel over time, especially if applied too thickly or if the wood expands and contracts with changes in humidity. Wood moves, and the coating needs to move with it.
Some finishes function as hybrids, combining elements of both penetrating and surface finishes. Hard wax oils and Danish oil, for example, both soak into the wood and leave behind a thin, protective layer that enhances durability while still allowing the wood to breathe. These finishes offer a balance between natural aesthetics and resilience.
Polymerization: How Oils Harden Over Time
Some oils undergo polymerization, a process in which they harden into a semi-solid state. This happens when the oil absorbs oxygen from the air, triggering a chemical reaction that forms a durable, water-resistant surface. This reaction is what makes drying oils like tung oil and linseed oil so effective as wood finishes. Linseed oil takes 24-48 hours to absorb into wood and 2-4 weeks to polymerize. Tung oil takes 24 hours to absorb and 2-4 weeks to fully polymerize. Over time, these oils develop a strong yet flexible film that protects the wood while still allowing it to breathe. At a molecular level, polymerization is simply crosslinking—individual molecules, known as monomers, bond together to form long-chain polymers, creating a more stable and resilient structure.
Non-polymerizing oils, such as mineral oil, coconut oil, and olive oil, never harden. Instead, they remain liquid, offering less permanent protection. While these oils are great for food-safe applications like cutting boards and wooden utensils, they require frequent reapplication and do little to protect against moisture or damage.
Overall Finish Comparison
Since there are so many different wood finishes, the easiest way to illustrate their differences is to show a table.
Sustainability of Wood Finishes
The sustainability of a wood finish depends on its ingredients, durability, and environmental impact. Natural finishes, made from plant-based oils, waxes, or insect-derived resins, tend to be more eco-friendly than their synthetic counterparts. They come from renewable sources, contain little to no harmful solvents, and biodegrade naturally when they wear away. Synthetic finishes, on the other hand, often rely on petroleum-based chemicals, release high levels of VOCs (volatile organic compounds), and require energy-intensive processing.
Durable But Eco-Sucky Finishes
Polyurethane, epoxy resin, and traditional varnishes make for very durable coatings but often carry some toxicity and are synthetically derived. Oil-based polyurethane forms a tough, plastic-like surface that resists scratches and spills, but it’s derived from fossil fuels, emits high VOCs during application, and can be difficult to dispose of properly. Epoxy resin is even less sustainable—it creates an ultra-durable, waterproof barrier, but it’s entirely synthetic, energy-intensive to produce, and virtually impossible to recycle. Traditional varnishes, such as spar varnish and alkyd-based coatings, offer great protection against the elements but contain solvents and synthetic resins that contribute to air pollution and hazardous waste. A respirator should be used when applying these coatings, and they are definitely not safe for food contact.
It should come as no surprise that some of the most durable coatings are not great for the environment because their purpose is to protect something that would naturally decay from the environment. As a result, these finishes don't break down easily or at all, can't be recycled, and can be toxic.
Sustainable and Non-Toxic Finishes
Among the most sustainable options, tung oil stands out as one of the best. Derived from the seeds of the tung tree, it penetrates deeply into the wood, polymerizes into a protective layer, and is completely biodegradable. It takes longer to dry than synthetic finishes, but once cured, it provides long-lasting protection against water and wear. Just watch out for unhealthy additives.
Similarly, raw linseed oil, extracted from flax seeds, is another excellent natural option. It soaks into the wood, hardens over time, and enhances durability without the need for synthetic additives. However, boiled linseed oil—often sold as a faster-drying alternative—contains chemical drying agents that make it less environmentally friendly.
Beeswax and carnauba wax come from bees and the leaves of the carnauba palm tree, respectively, offering a natural way to protect and polish wood. They don’t polymerize, so they require frequent reapplication, but they’re completely biodegradable and food-safe, making them ideal for applications like cutting boards and high-contact surfaces like wood table tops and other furniture. Shellac, a resin secreted by the lac beetle, is another eco-friendly choice. While it doesn’t provide the strongest protection against water, it’s a renewable, biodegradable finish that has been used for centuries to enhance fine furniture.
Hard wax oils provide a good blend of properties. Made from a blend of natural oils and waxes, they provide the penetrating effect of the oil with the surface coating of the wax, which offers good water resistance while maintaining a breathable, non-toxic finish.
Another alternative is pigmented oil finishes, which combine linseed or tung oil with natural earth pigments to add UV protection without resorting to synthetic additives.
Solvents
Solvents are liquids used in wood finishes to thin coatings, dissolve resins, speed up drying times, and clean tools. Traditional solvents like mineral spirits, lacquer thinner, and acetone are petroleum-based and emit high VOCs that can be toxic. Eco-friendly alternatives offer safer options without compromising performance. Citrus solvent (d-limonene), derived from orange peels, is a biodegradable substitute for mineral spirits, commonly used with tung oil and linseed oil. Pure gum turpentine, distilled from pine resin, works well with oil-based finishes and varnishes. Water, the cleanest solvent, is used in water-based polyurethane, acrylics, and stains. Low-VOC, biodegradable solvents reduce environmental impact and improve indoor air quality.
Responsible Use of Synthetic Finishes
While many of these non-toxic, naturally derived wood finishes offer excellent protection, there still may be cases where synthetic coatings are the best protection. Marine spar varnish, for example, is the most durable coating for boats exposed to saltwater and UV.
Even when using less eco-friendly finishes, there are ways to minimize environmental harm. Water-based polyurethane instead of oil-based versions reduces VOC emissions, and low-VOC varnishes help limit air pollution. Proper disposal is also key—many synthetic finishes require hazardous waste handling and should never be poured down the drain, on the ground, or thrown in regular trash. These should be brought to hazardous waste collection sites.
Branding and Products
Now that we've established all the main types of wood finishes and their properties, sustainability, and toxicity, things get complicated when choosing a specific product. Many wood finish products are combinations of several different chemicals and have different additives. Names of finishes are thrown around interchangeably by manufacturers, retailers, and woodworkers. Confusion is understandable when someone says shellac instead of varnish or acrylic instead of polyurethane. Most brands don't offer ingredient lists (and are not required to), claiming a proprietary formula. Usually, only the brands with healthy, sustainable products will list their ingredients.
Many finishes use vague terms like "polyurethane blend." Sometimes, a product will be labeled something like "tung oil finish" to lead people to believe it's a natural product when really it's a synthetic one with high VOCs and a little tung oil in it.
So, how do you know what a wood finish product is composed of and if it contains harmful ingredients? Unfortunately, you can't always know for sure, but there are a few things to look for. Certifications such as Green Seal and Greenguard can help identify low-VOC products. Looking at the product's SDS (safety data sheet) can help reveal known toxins and VOCs. If it's certified "food-safe," that's a good indication of overall safety. The best-case scenario is if the full ingredients list is available.
Moving Toward Sustainable and Healthy
With any chemical product, it's tempting to ask why the synthetic toxic products are so widespread while the safer alternatives are niche products for hippies. The dominance of synthetic, toxic wood finishes over natural, eco-friendly alternatives is largely driven by cost, performance, and industry inertia. Petroleum-based finishes like polyurethane, varnish, and epoxy are cheap to produce, fast-drying, and highly durable. They're attractive for mass production and commercial use. In contrast, natural finishes like tung oil and hard wax oil take longer to cure, require more frequent maintenance, and are often more expensive due to sustainable sourcing and smaller-scale production.
Additionally, marketing and regulation play a role. Synthetic finishes became the industrial standard in the mid-20th century, and large chemical companies promoted them as superior. Safer alternatives were largely sidelined as "niche" or "boutique" products, despite their long history of effectiveness. Now, with growing consumer demand for low-VOC, non-toxic options, natural finishes are making a comeback, slowly shifting the industry toward sustainability. Companies are figuring out ways to combine natural oils and resins with natural mold inhibitors and insect repellents for better outdoor durability.
If we're going to build things out of renewable materials like wood, we need to protect and preserve them with non-toxic, non-polluting, sustainable finishes.
Questions for you:
Do you think about the sustainability and toxicity of wood finishes or just how it looks?
Should manufacturers be required to disclose all ingredients in their wood finishes?
Would you be willing to pay more for a wood finish that is safe, sustainable, and long-lasting?
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