This article was inspired by a question from K Sanchez who writes: “What is the the difference between spar varnish and regular varnish?”
Why We Need Outdoor Formulations
Before we dig into the details, let’s talk about why we need a different varnish for outdoor applications. Any wood stored outside is going to be exposed to a wide range of temperatures and weather, as well as a good dose of damaging UV rays. These elements serve to break down the finish over time. Furthermore, changes in humidity cause the wood to expand and contract, and a standard indoor finish would simply crack and deteriorate under these conditions. Spar varnishes are typically designed to not only protect the wood, but also give it the flexibility and UV protection it needs to last for years. And the name “spar varnish” comes from the boating world, where the long wooden poles that support the sails are know as spars. So a spar varnish needs to be one that can withstand the rigorous conditions of seafaring life.
Nearly all modern varnish contains a few basic components: oil, resin, and a solvent. By modifying the types and amounts of these components, we can create a whole range of mixtures that vary in price and are specifically suited for either indoor or outdoor use. Fortunately for the inquisitive finisher, there are only so many ingredients that manufacturers have to choose from. And this makes it easier to see beyond the marketing jargon to deduce how a particular finish will behave. Here are the most common recipe ingredients:
Oils – Linseed Oil or Tung Oil
Resins – Alkyd, Phenolic, or Polyurethane
Solvents – Mineral Spirits, Naptha, or Paint Thinner
Oil to Resin Ratio
When a varnish is made, the ratio of oil to resin can have a dramatic effect on the way the varnish will behave. For instance, using a small amount of oil and a large amount of resin will produce a very hard but somewhat brittle finish. Obviously, this is not suitable for outdoor applications since we need an outdoor finish to be flexible. So what makes more sense is to create what is known as a “long-oil varnish”, that is, a formulation that contains a greater percentage of oil. The extra oil results in a softer, more flexible finish that will not crack when the wood expands and contracts.
The most common oil used to make varnish is linseed oil. Its lower cost makes it the most practical choice for both indoor and outdoor formulations. But many believe that tung oil is actually better for outdoor use. After all, a higher quality oil should equate to a higher quality varnish, and thus a higher price tag. As a result, many of the high-end marine varnishes will be made with tung oil instead of linseed oil.
Generally speaking, phenolic resins are best-suited for outdoor use. But that doesn’t mean every spar varnish is made with phenolic resins. Much like the situation with oils, the better product is also the most expensive. So you’ll find plenty of outdoor formulations using alkyd and urethane resins. A popular finish like Helmsman Spar Urethane contains urethane modified alkyd resins. A higher quality finish like Epifanes contains phenolic modified alkyd resins. There are many other brands of outdoor oil-based varnish, but the ingredients list is usually much more revealing than the words on the front of the can.
Sun Block For Wood
Most spar/marine varnishes will contain other important additives, such as UV blockers, that give the wood that extra bit of protection it needs in harsh conditions. UV light will not only damage the wood, but also the finish itself, eventually resulting in finish failure. So its a good idea to use a finish containing UV-blockers for any outdoor project.
Generally-speaking, my preferred outdoor varnish would be a long-oil varnish made with tung oil and at least some phenolic resins and UV inhibitors. And most times the brand I reach for is Epifanes.
If you’d like to learn a little more about how I apply varnish, check out this article: A Better Way to Apply Spar Urethane
If you want to learn more about my favorite finishing method, you should check out my DVD, A Simple Varnish Finish. My goal with the DVD was to demystify finishing by going over each and every step of the wiping varnish process. Even someone new to the world of finishing can create a show-stopping finish if they follow the methods outlined in the DVD. So check it out!