Staining, waxes and oil finishes
Although staining timber is optional, applying a finish is essential to protect the wood’s integrity. Without a finish, wood can dry, crack and deteriorate or – if exposed to moisture – swell so that drawers and doors no longer work. A good finish prevents swelling and cracking, protects against stains and enhances the appearance of the wood.
Oiling offers good protection from dirt and permeates timber to bring out its natural characteristics and colours.
Timber oil is resistant to water and alcohol and can be used for interior and exterior purposes. It is particularly good for exterior fittings such as timber cladding, due to its ability to prevent damage from moisture. It contains little to zero quantities of pigments which works to preserve the colours. It also provides a natural finish and is easier to fix than many other methods, if necessary.
However, oil is not particularly durable as it needs to be reapplied periodically. First-time application is also quite difficult because coating needs to be done multiple times before full absorption.
The true oils — Linseed oil and tung oil, the drying oils most often used in finishing, are readily available and relatively inexpensive. These finishes are called true oils to distinguish them from other products hyped as oil finishes and to separate them from naturally nondrying or semidrying oils used in finishes, such as soybean oil. These true oils change from a liquid to a solid through polymerization, a process that strengthens the cured finish.
Wax is a translucent decorative finish often used for protection of timber and has the added advantage of dual uses: it can be used on its own or over the top of another finish.
It is easy to apply and provides a great aesthetic look. One major attribute of wax is heat-resistance, and it can be combined with oil to give formidable protection, which is especially useful for hot or sunny regions. Wax is ideal for indoor use when used alone.
Lacquer is a thin, highly durable finish that provides terrific protection against dirt, water, and many other substances. It dries very fast, meaning a shorter coating time. It is most usually sprayed, which means you will have to hire or buy a spray system. It’s easy to repair when damaged. Timber should be appropriately sanded and cleaned immediately before lacquer application.
Nitrocellulose lacquer is the most common. If the label on the can says lacquer, it’s most likely nitrocellulose, which is made from an alkyd and nitrocellulose resin dissolved and then mixed with solvents that evaporate quickly. This type of lacquer has moderate water resistance
Shellac is a resin made from lac bugs and is one of the easiest finishes to apply. Just like lacquer, it dries quickly and can be applied by brushing or spraying. It polishes well and is quite durable. Shellac is also easy to fix when damaged.
Varnish is made of tough and durable synthetic resins that have been modified with drying oils. Labels on cans of varnish will list resins such as alkyd, phenolic and urethane, and the oils used are tung and linseed, as well as other semidrying oils such as soybean and safflower. Varnish cures by the same process as true oils — polymerization — but the resins make this finish more durable than oil. In fact, oil-based varnish is the most durable finish that can be easily applied by the average woodworker. Varnish surpasses most other finishes in its resistance to water, heat, solvents and other chemicals.
Water-based finish contains some of the same ingredients as varnish and lacquer — notably urethane, alkyd and acrylic — but many flammable and polluting ingredients have been replaced with water. The chemistry in this product is complex. Because the resins don’t have a natural affinity for water, they must be chemically modified or forced to combine with water.
Water-based finish is usually made with either an acrylic resin (sold as water-based lacquer) or an acrylic urethane mixture (sold as water-based polyurethane). As with varnish, the addition of the urethane makes the resin tougher and more scratch resistant, but water-based urethane does not have the same solvent and heat resistance as its oil-based counterpart.