Spar varnish is a hard finish created for outdoor use and on boats. It shields wood from the sun, rain and harsh weather. That is why it is so often used on boats, outdoor furniture, porch swings, railing and decks.
While standard varnish or polyurethane will harden and crack, spar varnish is formulated to remain flexible. Wood contracts and expands with changes in heat or climate. Spar varnish flexes with the wood, so it does not crack or peel. This is what makes it an ideal option for wood that is kept out all year.
Boat builders and outdoor woodworkers prefer spar varnish because it gives wood a shiny, long-lasting, yacht-style finish.
What Makes Spar Varnish Different ?
Spar varnish was first used on sailing ships. Ship masts and booms, called “spars,” had to bend in the wind and survive harsh weather. The finish needed to stretch without breaking.
Spar varnish is composed of natural oils, strong resins and UV blockers. The result is an soft and flexible coating that can stretch with the wood. Regular polyurethane is harder and may crack in the cold or hot weather.
Today, spar varnish still has UV inhibitors that slow the sun’s damage. This serves to inhibit fading, chalking, and peeling.
Also read:
Stain vs. Varnish: Which Should You Use?
Marine Varnish: Your Wood’s Ultimate Defender
Best Woods to Use with Spar Varnish
Some woods work especially well with spar varnish.
- Teak – Very durable and water-resistant. Great for boats.
- Mahogany – Rich color and smooth finish.
- Cedar – Needs UV protection; spar varnish helps a lot.
- Oak – Strong but needs sealing to stop stains.
- Pine – Very absorbent; seal first before varnishing.
Before applying varnish, sand the wood smoothly and let new wood dry fully. This helps the finish last longer and look better.
Spar Varnish vs Spar Urethane
Spar varnish and spar urethane are both wood finishes made to protect wood outdoors. They do the same job, but they are different in how they are made, how flexible they are, and how they perform. Spar varnish is more traditional and very flexible, while spar urethane is tougher and dries faster.
Key Differences
| Aspect | Spar Varnish | Spar Urethane |
|---|---|---|
| Composition | Oil-based finish made from natural oils like tung or linseed oil | Made with urethane resins; can be water-based or oil-based |
| Flexibility | Very flexible and moves easily as wood expands or shrinks | Less flexible, but harder and more durable |
| UV Protection | Excellent UV protection for sunlight exposure | Good UV protection, but may wear faster in strong sun |
| Water Resistance | Strong protection against moisture and rain | Even better water resistance and chemical protection |
| Drying Time | Dries slowly and needs several coats | Dries faster; water-based types are easy to clean |
| Best For | Boats and wood exposed to harsh weather | Outdoor furniture, doors, and indoor/outdoor use |
| Maintenance | Needs sanding and recoating over time | Needs less maintenance and resists wear better |
How to Apply Spar Varnish (Step by Step)
- Clean the wood
Remove dirt, mildew, and old finish. Rinse well and let dry. - Fix damage
Fill cracks or holes and sand smooth. - Seal the first coat
Thin the first coat slightly so it soaks into the wood. - Apply thin coats
Brush on thin layers. Let each coat dry before the next. - Light sanding between coats
Lightly sand to help the next coat stick. - Final smoothing
Light sanding at the end gives a smooth, glossy look.
Best conditions: warm weather, low humidity, and no overnight dew.
How Long Spar Varnish Lasts and How to Maintain It
A good spar varnish job usually lasts 2–3 years outdoors before it needs touch-ups. Areas in full sun may need care sooner.
Maintenance tips:
- Lightly sand and recoat before cracks appear
- Clean surfaces yearly
- Store outdoor furniture properly in winter
With regular care, spar varnish can protect wood for many years.
Spar Varnish vs Other Wood Finishes
Here is a simple comparison of spar varnish with other finishes:
| Finish | Flexibility | UV Protection | Outdoor Life | Maintenance | Cost |
| Spar Varnish | Excellent | Very good | 2–4 years | Medium | Medium |
| Epoxy | Low | Medium | 5+ years | Low | High |
| Teak Oil | High | Low | 6–12 months | High | Low |
| Polyurethane | Medium | Medium | 1–3 years | Medium | Low |
| 2-Part Urethane | Good | Excellent | 3–5 years | Low | High |
Common Mistakes and Easy Fixes
- Brush marks – Sand lightly and recoat
- White haze – Caused by humidity; apply in dry weather
- Fish-eye spots – Clean surface before coating
- Cracking – Too many thick coats; always apply thin layers
Always work in a well-ventilated area and allow enough drying time.
Safety and Environmental Tips
- Wear gloves and eye protection
- Be sure to ventilate well while applying oil-based varnish
- Store used rags safely to prevent fire
- Water-based spar varnish is safer and more user friendly indoors
Opt for sustainable wood if you can.
Final Thoughts
The best finish for wood that’s been exposed to the weather is spar varnish. It resists, flexes and makes wood look beautiful season after season. When prepped and cared correctly, it makes outdoor wood look great for decades.
FAQs
What is the difference between spar varnish and regular varnish?
Spar varnish is flexible and UV-resistant. Regular varnish is harder and meant for indoor use.
How many coats of spar varnish are needed outdoors?
You will normally get the best protection out of 6–8 thin coats.
Does spar varnish turn yellow over time?
Very slowly. UV blockers help reduce yellowing.
Can spar varnish be used indoors?
Yes, on doors and windows, but ensure good ventilation.
Is spar varnish waterproof?
It is resistant to water, but not completely waterproof. Regular maintenance is needed.
