![]() But many believe that tung oil is actually better for outdoor use. Its lower cost makes it the most practical choice for both indoor and outdoor formulations. The most common oil used to make varnish is linseed oil. The extra oil results in a softer, more flexible finish that will not crack when the wood expands and contracts. 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. Obviously, this is not suitable for outdoor applications since we need an outdoor finish to be flexible. ![]() For instance, using a small amount of oil and a large amount of resin will produce a very hard but somewhat brittle finish. When a varnish is made, the ratio of oil to resin can have a dramatic effect on the way the varnish will behave. Solvents – Mineral Spirits, Naptha, or Paint Thinner Oil to Resin Ratio Resins – Alkyd, Phenolic, or Polyurethane Here are the most common recipe ingredients: And this makes it easier to see beyond the marketing jargon to deduce how a particular finish will behave. Fortunately for the inquisitive finisher, there are only so many ingredients that manufacturers have to choose from. 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. Nearly all modern varnish contains a few basic components: oil, resin, and a solvent. So a spar varnish needs to be one that can withstand the rigorous conditions of seafaring life. And the name “spar varnish” comes from the boating world, where the long wooden poles that support the sails are know as spars. 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. Furthermore, changes in humidity cause the wood to expand and contract, and a standard indoor finish would simply crack and deteriorate under these conditions. These elements serve to break down the finish over time. ![]() 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. 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 Formulationsīefore we dig into the details, let’s talk about why we need a different varnish for outdoor applications.
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