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Originally developed for use on boats, ‘spar varnish’ now refers generally to any clear outdoor varnish. This article explains when, why, and how to use spar varnish on your next project. After reading, you’ll be ready to protect wood for use outside with a gorgeous and durable spar varnish finish.
Why Should I Apply Spar Varnish?
Anything built with wood must take expansion and contraction into account. To get technical for a moment: wood is hygroscopic. That means the moisture in the wood decreases and increases in accordance with the relative humidity of the surrounding air.
When wooden furniture is used inside, it experiences only a small range of humidity, due to climate control and protection from the elements. The expansion and contraction effect is therefore small.
Outside, where the climate is not controlled, wood is subject to frequent and drastic expansion and contraction. Inevitably, this damages the wood. Therefore, any wood that will be placed in a non-climate controlled space needs a protective barrier.
Spar varnish is the strongest clear coat wood finish for outdoor environments. It will keep you wood protected from sun, rain, wind, and the other outdoor elements.
What Is Spar Varnish?
All varnish is made up of three basic ingredients: oil, resin, and a solvent. Spar varnish originally referred to a flexible, elastic protectant applied to the spar mast of sailing ships. Today, it is used as a generic term for any outdoor varnish.
Linseed oil is the most popular varnish oil base. The other option is tung oil, which is significantly more expensive. ‘Long-oil’ refers to varnishes with nearly equal parts of resin and oil. ‘Short-oil’ varnishes use less oil, creating an extremely hard protective layer, however the finish will be less durable. Most spar varnishes today are long-oil formulations.
There are three types of resins most commonly used to make spar varnish.
- Phenolic resin, derived from formaldehyde, is the most common.
- Alkyd, a polyester resin, is also popular.
- The third kind of resin you’ll see used in spar varnish is urethane.
Phenolic resin tends to yellow overtime, while alkyd and urethane resin-based spar varnishes should remain clear.
Without a solvent, the resin-oil blend would be too thick to spread on wood. The most common solvents used are paint thinner and mineral spirits.
You can add more solvent for a thinner varnish, useful for sealing the wood and building the varnish base. As you add layers of varnish, less solvent should be used. For the final layers, which should be the thickest, very little or no solvent is necessary.
How to Apply Spar Varnish
Professional varnishers are well-compensated for their knowledge and skill, but you don’t have to be a pro to protect your workpiece. Follow these steps and you’ll have a thick protective coat of spar varnish on your workpiece.
- Choose your brush. You don’t have to use a new brush, but make sure the one you choose is clean and has never been used for paint. Badger hair brushes are the choice of varnishing pros, but they are expensive and must be carefully cleaned. Any natural bristle brush will do. Buy several. When the brush becomes overloaded and heavy, place it into a cup of paint thinner and switch to a new brush.
- Choose your solvent. Mineral spirits, naphtha, or paint thinner can all be used to control the viscosity of spar varnish. Use whichever you prefer, or whichever you have on hand. Naphtha evaporates more quickly than other solvents, so it may slightly reduce your dry time.
- Choose your resin. The most important thing for your spar varnish to have is good UV absorption, to protect the wood from sun damage. If you don’t want yellowing, choose an alkyd or urethane resin-based spar varnish. If an amber glow would enhance the look of the wood, phenolic resin is the way to go.
- Choose your oil. Tung oil is revered by mariners for the exceptional water protection it provides. However, tung oil-based spar varnishes are considerably more expensive than their linseed oil-based counterparts. For the extra investment, tung oil gives a harder, more durable finish with better water protection. For boats and uncovered patio furniture, it’s probably worth it to splurge on tung oil based-varnishes. For furniture that lives outside but is covered from the weather, linseed oil is perfectly adequate.
- Wait for the right conditions. Don’t varnish outside on a windy day. The wind will blow dust, dirt, and sand into your wet, sticky varnish. For spar varnish to set properly, the air temperature must be between 50 and 80 degrees. For the best results, wait until the forecast shows a few warm, calm days in a row, as spar varnish typically takes 24 hours to fully cure. With too much moisture in the air, the varnish will not cure. The ideal atmospheric condition would be 45 to 50 percent relative humidity.
- Prepare your workspace. Set up a light to shine horizontally across the surface of the wood. This will help you detect any missed spots as you apply the varnish. If you are varnishing boat trim, use plastic sheeting and masking tape to protect non-wood areas of the boat. Patio furniture or smaller items should be placed on a drop cloth. Spar varnish fumes can cause nausea; choose a well-ventilated area.
- Prepare your surface. Inspect the wood. Is it brand new? If so, start by sealing it against moisture with three coats of epoxy. Is there brittle, faded, or old applied varnish? Remove it with a chemical stripper. Alternatively, you can sand it away with sandpaper. Start with 180 grit paper and work your way up to 400 grit.
- Pour the spar varnish into a dedicated container. Never apply spar varnish directly out of the can. Instead, pour it into a new or very clean container. The brush can pick up sawdust and other material as you are applying the spar varnish, contaminating the entire can.
- Mix your varnish. For the base coats, a thinner spar varnish is preferable — about ¼ solvent to ¾ varnish. Add the solvent of your choice, then stir very gently and only until mixed. As you add coats, you can thicken the mixture by adding more varnish. The final coats should be nearly full-thickness spar varnish, with little or no thinner added.
- Avoid bubbles. Never shake or vigorously stir varnish. If you’re not adding thinner, it doesn’t need to be stirred at all. When adding thinner, mix it in slowly and gently with a stir stick. Shaking or stirring varnish introduces air to the mixture, which causes bubbling. If you do get bubbles on your workpiece, you will need to sand them down and apply an additional coat.
- Load the brush with spar varnish. Dip it into the dedicated container. Stop when the bristles are submerged about halfway. Allow the brush to become saturated, and then lift it out of the varnish, allowing excess to drip back into the container.
- Apply the varnish. When the brush is no longer dripping, apply it to the wood. Use long, even, and smooth strokes, brushing in the same direction as the grain. Avoid applying excess varnish — the coat should be relatively thin. Stop when the entire surface is covered in the first layer of varnish and check your work.
- Wait for the varnish to cure. Early coats will take 24 hours. As the varnish builds, you may need to wait 48 hours between coats. If you encounter a gummy spot while sanding, that’s an indication that the varnish didn’t dry completely and you will need to wait. Cover your brushes with plastic wrap and stick them in the freezer while the varnish is drying. This will keep the varnish wet, so you don’t have to clean the brush between layers.
- Sand the varnish between coats. Sanding the varnish will remove any ridges of accumulated material. It also provides a grippy surface for the next layer of varnish to attach itself to.
- Add as many layers as you like. It’s common for professional boat varnishers to add ten or more coats of varnish. Six to eight is usually sufficient for wood that is being varnished for the first time — you’ll add more during routine maintenance. Every six months or once a year, sand down the top layers of varnish and add a few new coats. This will stave off the need for a full re-varnishing.
Spar varnish is a flexible protective coating applied to wood that will be used outdoors. It protects wood from damage due to UV rays, moisture or changes in temperature/humidity. This kind of varnish can be applied using a foam or bristle brush.
Use paint thinner to make the varnish easier to spread. The number of layers you will need to apply depends somewhat on the thickness of your varnish. Slowly building up the spar varnish gives you the best results — try to be patient!
Varnish must be completely cured before sanding; this usually takes about 24 hours. Uncured varnish will be sticky or gummy to the touch. Sand between each layer for the best outcome.