If you are finishing wood with a natural or stained appearance today, you’re likely using some kind of varnish or polyurethane. Varnishes create a hard finish that protect and beautify everything from wood floors to delicate woodwork, but do you know the difference between a spar varnish and regular varnish and when to use them?
Wood finished for outdoor use is subject to a completely different set of factors that indoor finishes never have to deal with. Huge temperature swings, big changes in humidity, and punishing UV rays are the primary effects your finish will have to endure. These elements will easily break down a regular varnish or polyurethane over time.
Humidity changes, for one, cause the wood to swell and shrink much more than wood left indoors, and that excess movement alone is enough to crack a regular varnish in short order. So, for an exterior wood finishing solution, we turned to boat makers and their years of experience in caring for wooden elements with the harshest exposures in the world.
What is Spar Varnish?
The term “spar varnish” comes from the boating world, where the long wooden poles that support the sails are known as spars. A spar varnish is a finish specifically designed withstand the rigorous conditions of seafaring life, which means it can also handle anything your backyard throws at it.
So, what’s in it? Well, every varnish or polyurethane is made up of relatively similar ingredients. Just like with the grocery store, if you can cut through all the marketing hype and look at the actual ingredients, you’ll be able to make a better decision.
They all contain, in varying amounts, an oil, resin, and solvent. Manufacturers like to play around with the amounts and kinds of each to make their varnish, but this basic recipe is always followed. Within those three ingredients, you also have a couple options.
Linseed oil and Tung oil are the most commonly used oils in varnish and they create a deep penetrating finish. If your finish were only oil, then you’d have a very slow drying and soft finish that penetrates deep within the wood. Using oils alone is an option, but it takes many coats to build up a thick enough coat to protect the wood. These oils are an important part of a good spar varnish, because by adding more oil to the mix, you get a softer and more flexible finish that can handle the excess movement associated with outdoor wood. Higher oil content is one of the markers of a good spar varnish.
Resins are the hardeners. Typically, the resin is either alkyd, phenolic, or polyurethane. Alkyd and phenolic are associated with varnish finishes, while the use of a polyurethane resin is what makes a finish a polyurethane. These ingredients give us the hard, shell-like finish you are accustomed to in most varnish finishes. Interior finishes are relatively high in resin content and low in oil content, which creates a nice hard finish, but leaves little flexibility and penetration. Without the resin, you’re left with an extremely soft finish that may not hold up to traffic and wear.
These resins and oils need a carrier to be dissolved in, and that is what the solvent provides. There are a multitude of solvents, but mineral spirits and paint thinner are the most common. There are others like naphtha and xylene that are faster drying as well. The solvent thins the oils and resins and helps blend them together and make them easy to apply with a brush or wiping cloth.
Last but not least, every spar varnish needs to protect itself and the underlying wood from the sun. Regular polyurethane and varnish contain little, if any, UV blocking additives, and that alone makes them a poor choice for exterior application. Without UV protection, varnishes and wood quickly break down and fail.
Selecting a Varnish
Like most things in life, you get what you pay for. Better ingredients result in a better product and those better ingredients cost more. Will it kill your project to use a cheaper finish? Absolutely not, but considering the amount of work you put into your project, wouldn’t a few extra dollars be worth it for a longer lasting finish?
For me, there are two options I usually go to for exterior wood finishing. Both are excellent choices, though one does stand above the other.
This spar urethane is probably the most readily available spar urethane on the market today. You’ll find it in just about every big box and local hardware store. According to the ingredients we discussed earlier, Helmsman uses a polyurethane resin, hence the name and the primary solvent is mineral spirits. It is easy to apply and creates a nice warm finish that stay flexible for years. You can find it in a variety of gloss levels as well.
You won’t find this on your local hardware store shelves, but that doesn’t mean it’s not incredible stuff. Personally, this is my favorite finish for exterior wood. It was designed by boat finishers for boats and us land lovers can benefit from their hard work by using a product that can handle the toughest elements. It uses a phenolic resin, which I think is better suited to outdoor use, and the solvent is naphtha with a little bit of xylene. You can usually only order it online. If I’m finishing an exterior wood project, 9 out of 10 times Epifanes is the stuff I reach for.
What it all comes down to is protecting wood, and for exterior applications, spar varnish does a better job of it plain and simple. Just like you need the right tool for the right job you also need the right varnish for the right application. Finding the right varnish is only half the battle, though. You need to know how to apply it properly and we’ll be talking about that next week.
The main reason most finishes fail prematurely is not because of an inferior product, but because of user error. Poor preparation and application can’t be overcome by a premium product. So, tune in to learn how to get a great finish next week.
Founder & Senior Editor
I love old houses, working with my hands, and teaching others the excitment of doing it yourself! Everything is teachable if you only give it the chance.