Determining the Right Finish for Your Project

Although at least 10 varieties and more than a dozen brands of finish are available, all can be divided into two categories: penetrating finishes (those that dry inside the wood) and surface finishes (those that dry on the surface of the wood). Penetrating finishes are easier to apply and leave a more natural look. Surface finishes are more durable but don’t look as natural.

Types of Hardwood Finishes

PENTRATING SEALERS

A penetrating sealant soaks into the wood and fills the pores to protect it. In the past, this was always the favored method of treating wood but has more recently fallen out of favor with the advent of the wide variety of polyurethane finishes. Penetrating sealers can show more wear and tear than surface finishes and do need to be consistently oiled or coated with wax. Penetrating finishes don’t produce as high of a gloss of a surface finish.
Like so many finishes, you can’t simply add a layer of a different product if you want to refinish. You must sand down the wood to remove any sealant or wax before applying a new product. However, penetrating sealers are favored by many craftsman for showing the wood to its best advantage.

Tung Oil: Probably the most popular penetrating sealer, tung oil comes from tung tree nuts. Used for centuries by the Chinese, tung oil is easy to apply, has an amber hue, and doesn’t darken the wood as much as linseed oil. Tung oil does have drawbacks. It dries slowly and requires several coats (with sanding in between) to achieve a durable surface. It also tends to turn a whitish milky color if it is applied too thickly. However, as a natural product, it’s sustainable and is even food safe.

Linseed Oil: Together with tung oil, linseed is known not only as a finish and sealer but also a “drying oil.” Unlike solvent oils in surface sealers, the oil in tung or linseed absorbs air and hardens as it cures, in contrast to surface sealers that cure when the oils evaporate. Made from seeds taken from the flax plant, linseed is another natural product and also low in VOCs like tung oil.

There are some differences between these two oils. Linseed oil isn’t quite as water resistant as tung oil. In addition, linseed is slightly darker. Applied similarly, linseed and tung oils are not difficult to use as long as you can wait for the wood to dry afterward. Touch ups, to cover scratches, also work better than with other finish products.

Danish Oil: Danish oil is neither Danish nor a “true” oil – it’s a mixture of tung oil and varnish. Although varnish is classified as surface sealer, Danish oil penetrates instead. The result, however, is slightly different than either product would suggest. More durable than oil, more environmentally-friendly than varnish and offering a satin sheen, Danish oil dries fairly quickly and is considered food safe as well. It also requires fewer coats than drying oils, but the same reapplication and touch ups as needed are required.

SURFACE SEALERS

As the term implies, a surface sealer coats the top of the wood, creating a plastic-like barrier between the wood and the environment. The way it works is simple – a mixture of resin and solvent, a surface sealer begins to dry when applied to the flooring. What’s left, after the solvent evaporates in the process, is the resin, which hardens to form the coating. So when you touch a something that has been surface-sealed, you’re not touching the wood, but the surface sealer. This finish is very durable but can become worn on heavily used items or in high traffic areas, so occasionally it must be reapplied. Performed properly, surface-sealed wood remains water resistant and durable with a minimum of maintenance or cleaning. A lower level of maintanence is one of the main reasons this type of finish has become so popular.

Shellac: One of the oldest finishes available, in spite of its recent decline in popularity, shellac remains a particular favorite of some woodworkers. It’s easy to understand why. Shellac provides a pleasing gloss and is versatile and natural. Shellac is made from the droppings of the Lac bug, which feeds on trees in Thailand and India. Lac bugs then secrete the resin, called lac, in cocoon tunnels, in which it lays thousands of eggs. Workers scrape these encrusted branches and dry the lac into chips. When mixed with denatured alcohol, shellac is formed.

Shellac comes in several formulas. It can be tinted with custom color or used in its natural state, which is slightly orange. A bleached formula is clear, and another formula removes the naturally occurring wax content to allow shellac to be used in combination with other products. Shellac is inexpensive, dries quickly, doesn’t re-wet stain, doesn’t require sanding between coats and doesn’t yellow like oil-based varnish or polyurethanes. The fast drying time does demand confident application. Additionally, shellac isn’t as durable as some other surface sealers and is vulnerable to ammonia, alcohol and water spotting, even if it is contained in cleaning products.

Varnish: People have been using varnish since the days of the Ancient Greeks; the term “varnish” comes from “Berenice,” a city in modern-day Libya where this finish was purportedly first invented. Varnishes contain resins suspended in a solvent and blended with what is known as a drying oil. Drying oils are oils which will firm and become glossy when they are exposed to the air. The first varnishes were vegetable oil-based, but today’s varnish uses synthetic resins (vinyl-alkyds) instead.

Tougher to wear or penetrate than shellac, varnish is still not as durable as polyurethanes. However, it’s fairly inexpensive and can be tinted a variety of colors. Varnish is considered fairly high in volatile organic compounds (VOCs) due to the solvent content.

A major disadvantage with varnish is the difficulty in application. Similar to shellac, it takes something of a talent to apply. Unlike shellac, it dries slower, making dust and other contaminants a complication. Also, a varnish finish requires wet sanding between layers. Varnish can be thinned slightly with an oil to make application easier but thinning is a double-edged sword, as it also makes the finish softer.

Don’t let products labeled varnish confuse you. There are products called wiping varnishes and others called oil-varnish blends. Wiping varnish is thinned with paint thinner. This makes it somewhat easier to apply and allows some of the varnish to sink into the wood like a penetrating finish, while creating a buildup like a surface finish. An oil-varnish blend, in contrast, mixes varnish with tung or linseed oil. The result is a penetrating surface that acts – and is applied – like an oil, with no buildup when used on floor surfaces.

Lacquer: Not to be confused with shellac, lacquer is a fast-drying wood finish typically applied with a sprayer. It dries extremely fast, making dust and contaminants less of a problem, although slower-drying brushing lacquers are also available. The finish quality is glossy and fairly hard, but lacquer is high in VOCs. There are now some water-based lacquers currently on the market which have a lower VOC level.

Polyurethanes: For the highest gloss possible in a hard, durable finish, use a polyurethane product – also called urethane. This, along with the variety of urethane types, can become somewhat confusing, so it’s important to pay attention to labels when purchasing any polyurethane finish product to ensure you know what you are getting.

Of all the surface finishes used for hardwoods, urethane formulas are some of the most common today. Oil-modified urethane is particularly popular as it is fairly inexpensive and easy to apply, although it dries fairly slowly. Because it is solvent based, it will contain VOCs. It also has a tendency to turn an amber color with age, especially when exposed to bright sunlight.

A water-based urethane formula dries quicker than oil-based and is non-yellowing and low in odor. Water-based formulas are thinner than an oil-based finish so it takes additional coats. These formulations are a low VOC products.

A third urethane finish formula is the moisture-cure urethane. Available in either satin or gloss, in a non-yellowing formula or the regular amber-tinted finish typical of most penetrating sealers, moisture-cure is usually a little more expensive than other sealers such as shellac or varnish. The term moisture-cure refers to the fact that the humidity in the air helps this formula dry faster.

The last type of urethane finish, acid-cured, is called a Swedish finish due to its country of origin. Also known as a conversion varnish, it is a clear formula that won’t turn yellow and dries extremely quickly. High in fumes and extremely difficult to apply, it’s generally best applied by a professional, making it even more expensive than other urethanes.

Wax: It’s been around forever, and it’s likely not going anywhere soon. A wax finish soaks into the pores of the wood and hardens, like a penetrating finish, but leaves a layer on top like a surface finish. Often used in combination with a stain (and working with most stains easily) the result is a project with the color you desire with a low-luster, slightly amber-tinted finish.

Most waxes are applied as a paste and buffed to cure and shine. Low in odor and fairly durable, wax is vulnerable to water and some solvents as well as scratching. Since it’s fairly inexpensive, it’s the easiest to reapply as needed – if you overlook the elbow grease that goes into applying it.

So, now you’ve learned all there is to know about hardwood finishes… next, we turn our attention to the basics of applying them.

Sources:
http://tlc.howstuffworks.com/home/guide-to-furniture-finishes-ga.htm
http://www.realestate.com/advice/hardwood-floor-finishes-choosing-the-right-finish-for-your-floor-40010/
http://www.finewoodworking.com/toolguide/articles/selecting-a-finish.aspx
http://www.hardwoodinfo.com/articles/view/admirer/7/211

Tagged as: diy, finish, finishes, hardwood, lacquer, linseed, oil, polyurethane, sealer, shellac, tung, urethane, varnish, voc, wax, wood