Spar Varnish vs Spar Urethane? (Which One is Better?)

When looking for a sealant to use on your outdoor project, spar varnish and spar urethane are usually at the top of the list. But what’s the difference between both finishes and which is better?

The difference main difference is that spar varnish is made from a combination of wood oils, varnish, solvents, and UV protectors. On the other hand, spar urethane is made from a combination of urethane compounds, solvents, and varnish.

Spar urethane is a type of spar varnish. But, the presence of plasticizers, such as urethane, in it makes it stronger, harder, and more durable. Since it is more durable, it lasts longer on wood. On the other hand, spar varnish chips and flakes off gradually so you’ll have to re-apply it frequently.

Spar Varnish

What is Spar Varnish?

Spar varnish is a traditional wood finish that is made by mixing varnish, solvents, and wood oil, usually Tung oil or Danish oil. Additives like UV blockers, metalized driers, and resins are added to its formula to give the finish unique attributes.

Spar varnish was originally designed for the “spars” or poles of seal vessels. It was used to coat poles, bars, and spars to offer protection from water, scratches, and UV deterioration which is why it’s called “spar” varnish.

Today, spar varnish is no longer restricted to boats and ships as homeowners now use the finish on their exterior wood surfaces to protect them from UV rays, dents, and water damage. You can apply it with a paintbrush or an applicator rag. Paintbrushes are faster but applicator rags give a smoother finish.

Its downside is that the finish is not durable and you’ll need to reapply it on wood at regular intervals, usually every 12 months or less depending on usage.

You can use spar varnish:

  1. To protect the wood from water damage.
  2. Over bare wood.
  3. To protect exterior wood from UV degradation.
  4. On indoor furniture.

Spar Urethane

What is Spar Urethane?

Spar urethane is a modification of the traditional spar varnish. Woodworkers and sailors discovered that spar varnish isn’t very durable and the finish usually chips off the spars and poles after about a year or less.

This made them look for a way to make it last longer and the solution was to create spar urethane. Spar urethane has a similar formula to traditional spar varnish but spar urethane is reinforced with polyurethane compounds.

The presence of urethane compounds gives the finish increased strength, durability, and moisture resistance. You can only apply it using a paintbrush or paint sprayer if you are skilled enough.

You can use spar urethane:

  1. To protect the wood from water damage.
  2. For exterior surfaces.
  3. To protect the wood from scratches and dents due to its increased durability and strength.
  4. To shield wood from UV deterioration or fading.
  5. Over bare and painted wood surfaces.

Spar Varnish vs Spar Urethane

Here is a table that identifies the major differences between both finishes:

Spar VarnishSpar Urethane
The FormulaNaturalSynthetic
Wood ProtectionGoodBetter
Application AreasOnly on bare woodOn bare and painted wood
Applicator methodRags or paintbrushesPaintbrushes or paint spray guns

Next, let’s check out these differences in detail.

The Formula

Spar varnish has a natural formula while spar urethane has a synthetic formula. Spar varnish has a natural formula because a high volume of its ingredients is naturally made or gotten from natural sources.

On the other hand, spar urethane is a synthetic finish and some woodworkers refer to it as a mix of natural and synthetic formulas. It is synthetic because it contains man-made resins, plasticizers, and urethane compounds. 

Wood Protection

Spar urethane protects wood better than spar varnish because it is reinforced with urethane and plastic compounds. These plastics are in liquid form but as soon as the finish cures, the urethane compounds react with oxygen in the atmosphere to reveal an ultra-thick and hard finish.

This finish protects the wood from dents, scratches, and other factors that can cause wood damage. 


Spar urethane and spar varnish are very good at repelling moisture. Both finishes are specially designed to repel moisture and waterproof wood and they are equally good at this.

However, spar urethane maintains its waterproofing layer for up to 10 years. On the other hand, spar varnish will stop being waterproof usually after 12 months.

Application Areas

You can use spar urethane on bare and painted wood because the finish doesn’t penetrate the surface as deep. However, before you put it on painted wood, the wood must be fully dry, clean, and smooth. You can also use it on outdoor areas like fences and building exteriors.

On the other hand, spar varnish can be used on indoor and exterior surfaces. However, you can only use it on bare wood because it needs to penetrate the wood slightly to stick well. So, if there’s a finish on the wood already, spar varnish can’t penetrate it. To use it on painted wood, you’ll need to remove the paint first.

Applicator Method

Spar urethane can be applied using a bristled paintbrush or a paint sprayer. To spray the finish, you’ll have to thin it before loading it into the spray gun or suction cup.

On the other hand, spar varnish can also be applied using a paintbrush but you can’t spray it. Instead, you can use a clean applicator cloth to wipe it on the wood.

Final Words

In summary, spar urethane and spar varnish are great finishes that are ideal for blocking moisture and preventing water damage. You can also use either finish on exterior wood, but spar varnish is more suited for indoor wood surfaces.

Remember that spar urethane is stronger and more durable so you should use the finish on busy surfaces. 

Tony Adams
Tony Adams

Tony is a professional painter and an author of DIY Geeks. Tony has completed over 1,000 painting projects for his clients. It's safe to say he knows what he Is talking about,

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