WORKBENCH GUIDE
Oil, varnish or water-based finish?
Film finishes protect and clean easily; penetrating oils are simple to renew. How the Wood Handbook compares them indoors and outdoors.
Film-forming or penetrating
Finishes either form a film on the surface or penetrate the wood without forming one. Varnish, lacquer and paint build films; drying oils, nondrying oils and many sealers penetrate. Any clear finish fills the cells with something whose refractive index is closer to wood than air, which is why oiled or varnished wood shows deeper colour.
Indoors, film finishes such as varnish are generally the easiest to clean, resist staining better than oils or sealers and, when thicker, protect against scratches and dents; thicker, harder films suit floors. Oils leave the surface texture intact and are easy to renew, but keep dirt out less well than a film.
| Finish | Forms a film | Curing | In use |
|---|---|---|---|
| Varnish, lacquer | Yes | Cures; time depends on the product | Easiest to clean; can eventually crack, chip and peel |
| Sealers | No, penetrates | Cures quickly | Easy to apply; easy to clean, resists scratching |
| Drying oils (tung, linseed, walnut) | No, penetrates | May need several weeks | Easy to clean, resists scratching |
| Nondrying oils (vegetable, mineral) | No, penetrates | Never cures | Vegetable oils can go rancid; mineral oil does not |
| Paraffin wax | No | Solid at room temperature; apply warm | Does not go rancid; suits cutting surfaces |
Oils and wax
Drying oils such as tung, linseed and walnut oil cure, and the Wood Handbook notes that they may need several weeks to do so, while commercial sealers, often similar to thinned varnish, cure quickly. Nondrying oils, such as vegetable and mineral oils, penetrate but never cure.
Vegetable oils can turn rancid and feed mildew and bacteria. For cutting boards and butcher blocks, the Handbook points to food-grade mineral oil (liquid paraffin) and food-grade paraffin wax, which do not go rancid or support microbes; warming the wood to the melting point of the wax makes it easy to apply.
Water-based finishes
The Handbook describes advances in waterborne finishes driven by limits on volatile organic compounds, and advises against thinning modern waterborne paints. It gives a typical minimum of 10 °C during application and the following 24 hours for latex paints to form a proper film. Some formulations work at lower temperatures, so follow the product maker’s specified range.
Water in a finish raises the grain. The Handbook notes that coatings without water are actually more prone to cracking from grain raise later, because they do not release it during finishing: before a solvent-based finish, raise the grain with water and sand once it is dry.
Outdoors
Sunlight degrades the wood under clear films: the Handbook says the best exterior clear coats last about ten years when fully exposed, while a penetrating finish might last only two years but is quick to renew. On decks, film-forming finishes often peel, and penetrating stains or water repellents are easier to maintain.
A kitchen cutting board: food-grade mineral oil or food-grade paraffin wax rather than a vegetable oil. A dining table: a film finish such as varnish, over a surface sanded to the finish maker’s grit sequence.