By far the most common finish used when one wants to see the grain of the wood is varnish. While there are other finishes available, varnish has been used literally for centuries. Some modern varnishes are adaptations on the original, but they still qualify as being varnish. Many of these are labeled as “polyurethane varnish” rather than just saying “varnish.”
To understand the differences that these new varnishes provide, we need to have a basic understanding of the chemistry behind varnish. There are three basic ingredients in any varnish: resin, oil and a solvent. The solvent is there to allow the varnish to be brushed on and flow, with the intent that it dissolves. Therefore, it really isn’t all that important. I think we can all trust that the manufacturers will use a solvent that will accomplish the purpose.
It is the other two ingredients; resin and oil that make the difference in varnishes. The types of these ingredients used, as well as the ratio between the two of them, are what give each varnish its particular characteristics.
There are three types of resins used: phenolic, alkyd and polyurethane. In recent times, most varnishes are polyurethane, due to its lower cost, higher scratch resistance and clear coloration. The best (and most expensive) varnishes are made using phenolic. It is also possible that a combination of these resins will be used in one varnish.
There are two types of oils used in varnish: tung oil and linseed oil. Of the two, linseed oil is by far the more common, due to its lower price. However, many people feel that tung oil is a superior product, and it is found in the higher priced varnishes.
The other major distinction to look for in a varnish is the use that it is created for. Most varnish is created for indoor use, where the wood and the varnish on it are protected from weather and ultra-violet light. Spar varnish, sometimes referred to as marine varnish is created specifically for outdoor use. Since any wood left outdoors is much more susceptible to absorbing moisture and expanding, these varnishes have a higher ratio of oil to resin, making them more flexible. They also have UV inhibitors added to the varnish to protect it.
When selecting a varnish, it is important to consider the application; specifically, where the finished item will be used. If it is going to be used outdoors, then you definitely want a spar varnish. If it is going to be used indoors, then the next question is whether it will be subject to a lot of wear. A table top needs a more durable finish than baseboard and casing. Therefore, selecting a hard varnish, with good scratch resistance is important. Check out our list of the best vanishes in 2022.