What is Spar Urethane?


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Featured image: test piece of maple finished with spar varnish (Credit: wikimedia.org)

What is Spar Urethane?

The spar varnish name is related to the wooden poles on a marine vessel that offer support to the sails, the ship’s spars. This part of the vessel gets maximum exposure to wind, sun and moisture, so the wood needs an exceptionally high protection level. Due to this requirement, mariners developed spar varnish, which contains a high degree of solvents and resins. The resultant finish emerges harder and capable of protecting the wood from the sun’s ultraviolet (UV) radiation.

If you own or need to maintain a vessel, you can get spar resin in a liquid urethane plastic from the market. Because of the high resin content, spar varnish takes on a yellowish hue. Although it isn’t very suitable for indoor furniture, it serves as an excellent finish to outdoor furniture, decks and fences. You can also use it on indoor surfaces exposed to a high degree of moisture like bar tops, kitchen countertops, and shower seats.

The difference between spar varnish and regular varnish is that spar varnish contains a high level of solvents. It also has more additives meant to block UV radiation.

You can apply spar urethane or spar varnish-like any other regular varnish. However, because spar varnish is considerably thicker, you will find that it takes a longer time to dry than typical varnish, so there will be a more extended waiting period between coats.

Before and after pics: marine grade plywood wooden boat seats with Spar. (Credit: Woodworkingtrade)

Application Method for Spar Urethane

You can get the best results by applying spar urethane with a brush. You will find it too thick to spray, but you can thin it down with solvents if you need to spray it. The ambient temperature should be 50°F or above to apply spar urethane.

If you brush in the wood grain direction, you can avoid brush stroke marks from forming. Apply multiple thin coats rather than fewer thick coats for best results. You can also apply spar varnish over an existing coat of finish.

To get adequate adhesion, you must “scuff” the surface of the wood between coats. For this purpose, you can rub the wood down with 150-grit sandpaper.

Difference Between Spar Urethane and Polyurethane

Rust-Oleum 250241H Ultimate Spar Urethane Water Based, Quart, Satin Finish
Rust-Oleum Ultimate Spar Urethane

We use polyurethane to impart a smooth surface free of any blemishes on the surface. So, it is more about appearances. Conversely, spar urethane serves to protect wood exposed to the elements. It is also the preferred choice for coating the outer surfaces of boats that are to be submerged in water.

Urethane is malleable and flexible, making it ideal for applying on differently-shaped objects, and we use it in liquid form. Polyurethane exhibits stiffer and more rigid properties. Hence, we use it on firmer items. Spar urethane is much more durable than regular polyurethane because we use it on wood exposed to the elements.

Spar Urethane FAQs

Q: Is a spar urethane coating food safe?

A: All finishes are food-safe provided the curing is done correctly. The rule of thumb for a finish to become fully cured is to leave it on its own for 30 days at room temperature, more specifically between 65°F and 75°F.

Q: Is spar urethane waterproof?

A: Spar urethane renders wooden surfaces with water-resistant properties. It can be seen on hardwood flooring and furniture. We use spar urethane to protect the surface of spars of sails on sailboats exposed to the sun, wind and rain, so it needs to be extremely water and weather resistant.

Q: Should I sand between coats while applying spar urethane?

A: Nowadays, we use spar urethane on various wooden items and structures, both interior and exterior. The important thing is to do adequate prepping to the wood surfaces between each coat. Sanding between successive coats helps the next coat to stick well. It also helps to reduce the marks of brush strokes and bubbles.

Q: Can I get commercially-available oil-based spar urethane?

A: You can get an oil-based spar urethane which is crystal clear and specially designed for protecting wood against the forces of nature. In this type of finish, the oils help the expansion and contraction of the wood with the passing seasons.

Q: Which is better – water-based or oil-based spar urethane?

A: You will get ample protection from both water-based and oil-based spar urethane. Water-based spar urethane is best used on maple wood flooring. Oil-based spar urethane protects most wooden outdoor structures, including the upper parts of boats and ships.

Q: What are the drying times of spar urethane?

A: Drying and recoating times are related to the temperature. The best temperature for recoating is 70°F with a relative humidity of 50%. You will need to give more time if the temperature is lower. The coating will be dry to touch in about 2 hours, but you should wait for 4 hours to 6 hours before touching it and 8 hours before applying the next coat. You can use the finished coat for light use after 24 hours and full use after 72 hours.

Q: Does spar urethane turn yellow?

A: Oil-based spar urethane tends to turn yellow over time, but the water-based variety will not. Some top brands of oil-based spar urethane are guaranteed against yellowing. Other types may yellow slightly, and it can vary from brand to brand.

Q: Can we use spar urethane over paint?

A: You can use any good brand of spar urethane on a painted wood surface. It will act as a sealer and make the wood waterproof.

Conclusion

As you have seen, there is a lot of confusion between the different types of polyurethane and spar urethane. The main difference between spar urethane and other kinds of finish is the high degree of durability and water resistance that spar urethane has.

You can use spar urethane as a go-to finish for most outdoor wood surfaces and marine applications with considerable success. If you use spar urethane, although you may end up spending a bit more on your finish, your woodworking projects will be good to look at and highly durable.

Happy Woodworking!