Spar varnish is a type of wood-finishing varnish that was originally developed for coating the spars of sailing ships, which formed part of the masts and rigging. It is a finish specifically designed to withstand the rigorous conditions of seafaring life, which means it can also handle anything your backyard throws at it. 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.

The most important condition for such varnishes was resistance to flexing. This required a varnish that was flexible and elastic. Without elasticity, the varnish would soon crack, allowing water to penetrate the wood beneath. Prior to the development of modern polymer chemistry, varnish production was rudimentary. Originally, spar varnish was a "long oil" varnish, composed primarily of drying oil with a small proportion of resin, usually boiled linseed oil and rosin.

In modern times, "spar varnish" has become a genericized term in North America for any outdoor wood finish. Owing to modern varnish materials, their weather and UV resistance is likely to be good, but the original requirement for flexibility has largely been forgotten. A common form of modern spar varnish is spar urethane, a polyurethane-based finish intended for outdoor use, where sunlight-, heat-, and water-resistance are desirable qualities.

Some key components of spar varnish include:

• Oils: These 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: 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.

Spar varnish is recommended for use on exterior wood, fiberglass, coated metal, or painted surfaces. It can be used on unfinished wood surfaces or previously finished surfaces. This finish can also be used over stained surfaces that have thoroughly dried. It is important to apply spar varnish when air and surface temperatures are between 60° and 90°F and humidity is between 30% and 70% .