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 on the top of the list. But what’s the difference between both finishes and which is better?

The difference between spar varnish and spar urethane is that spar varnish is made from a combination of wood oils, varnish, solvents, and UV protectors while 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 like urethane in it makes it stronger, harder, and more durable than spar varnish. Since spar urethane is more durable, it lasts longer on wood. Spar varnish chips and flakes off gradually so you’ll have to reapply spar varnish frequently.

This post reveals more differences between spar varnish and spar urethane including when to use either finish. Let’s dive in.

What is 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 the formula of spar varnish to give the finish unique attributes.

Spar varnish was originally designed for the “spars” or poles of seal vessels. The varnish 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 spar varnish with a paintbrush or an applicator rag. Paintbrushes are faster but applicator rags give a smoother finish.

The downside of spar varnish is the finish is not very durable and you’ll need to reapply the finish on wood at regular intervals, usually every 12 months or less depending on usage.

What is Spar Varnish Used For?

  1. Spar varnish is used to protect wood from water damage.
  2. Spar varnish is used on bare wood.
  3. Spar varnish is used to protect exterior wood from UV degradation.
  4. Spar varnish is also used on indoor furniture since the finish is non-toxic.
  5. Spar varnish repels insects and pests from eating and living in the wood.

What is Spar Urethane?

What is Spar Urethane?

Spar urethane is a type of spar varnish. The finish is a modification of 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 spar varnish 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 in spar urethane gives the finish increased strength, durability, and moisture resistance. You can only apply spar urethane using a paintbrush or paint sprayer if you are skilled enough.

The downside to spar urethane is that the finish is slightly toxic. The synthetic formula and presence of urethane compounds in spar urethane mean that it emits low VOCs (volatile organic compounds) which can be toxic in large quantities. This is why spar urethane is ideal for exterior surfaces where the fumes and VOCs can be quickly dispersed.

What is Spar Urethane Used For?

  1. Spar urethane is used to protect wood from water damage.
  2. Spar urethane is ideal for exterior surfaces.
  3. Polyurethane protects wood from scratches and dents due to its increased durability and strength.
  4. Spar urethane contains UV blockers that shield wood from UV deterioration or fading.
  5. Spar urethane can be used on bare and painted wood surfaces.

So, is spar urethane better than spar varnish or is it the other way around? To find out the better finish, let’s compare spar urethane and spar varnish below.

Spar Varnish vs Spar Urethane

Here is a table that identifies the major differences between spar varnish and spar urethane:

 Spar VarnishSpar Urethane
The FormulaNaturalSynthetic
Wood ProtectionGoodBetter
Application AreasOnly on bare woodOn bare and painted wood
WaterproofingGreatGreat
Toxicity levelsVery low toxicitySlightly toxic
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.

Spar urethane on the other hand is a synthetic finish and some woodworkers refer to it as a mix of natural and synthetic formulas.

Spar urethane is synthetic because it contains man-made resins, plasticizers, and urethane compounds. All of these ingredients are made in laboratories giving spar urethane a synthetic formula.

Wood Protection

Spar urethane protects wood better than spar varnish. This is because spar urethane has been 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 varnish also protects wood but not as well as spar urethane.

Waterproofing

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 longer than spar varnish. Since spar urethane has been reinforced, it can remain waterproof for up to 10 years. On the other hand, spar varnish will stop being waterproof usually after 12 months.

Toxicity

Spar urethane is more toxic than spar varnish because spar urethane contains man-made chemicals, resins, and urethane compounds. The presence of these substances in spar urethane makes it slightly toxic.

Spar varnish on the other hand has very little to no levels of toxicity because it doesn’t have chemicals or man-made compounds in its formula.

Application Areas

You can use spar urethane on bare and painted wood because the finish doesn’t penetrate the surface as deep as spar varnish. However, before you put spar urethane on painted wood, the wood must be fully dry, clean, and smooth.

You can also use spar urethane on outdoor areas like fences and building exteriors but the finish isn’t the best fit for indoor surfaces due to its slight toxicity levels.

On the other hand, spar varnish can be used on indoor and exterior surfaces since it contains little to no levels of toxicity. However, you can only use spar varnish on bare wood because the spar varnish needs to penetrate the wood slightly to stick well and it can’t penetrate the wood if there is paint on it. To use spar varnish 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.

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

So, when should you use spar varnish and when is spar urethane the ideal choice? Let’s find out.

When Should You Use Spar Varnish?

You should use spar varnish when:

  1. You want to seal an interior surface – spar varnish is safer than spar urethane on indoor surfaces.
  2. Use spar varnish when working on bare wood or unfinished wood.
  3. Use spar varnish to protect wood from water damage.
  4. Use spar varnish when you want a wipe-on finish since you can wipe the finish directly on wood.
  5. Use spar varnish when you want a temporary finish because you’ll need to reapply the finish every few months if you want to use it as a permanent finish.

When Should You Use Spar Urethane?

You should use spar urethane when:

  1. You want to seal an exterior surface.
  2. Use spar urethane when working on busy surfaces like walkways and patios since the finish is stronger and more durable than spar varnish.
  3. Use spar urethane on bare or painted wood surfaces.
  4. Use spar urethane to protect the wood from damage, dents, and scratches.
  5. Use spar varnish when you want a permanent finish.

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 ideal for indoor wood surfaces.

Remember that spar urethane is stronger and more durable so you should use the finish on busy surfaces. Spar varnish on the other hand chips after a while so only use it on indoor or low-traffic areas.

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