What Is Spar Varnish and What Is It Used For?

Spar Varnish for Boats

If you need a durable and resilient wood finish, you have probably come across Varnish and Spar Varnish, besides other common choices such as lacquer, paint, polyurethane. While Varnish is generally popular for its UV resistance, you might be wondering what exactly the characteristics of Spar Varnish and the differences to the regular type are.

Spar varnish is actually an even more resistant type of varnish that is traditionally used for boats and ships. However, its improved protective effects compared to regular varnish is probably one of the reasons it is also chosen for finishing exterior wood and wooden installations. Read on to learn more about the characteristics, uses and pros and cons of Spar varnish.

What Is Spar Varnish?

Spar varnish is a special type of varnish used for finishing wood. Also sometimes referred to as Marine Varnish, it was originally designed for the marine sector for use on wooden parts of boats and ships. The harsh conditions that boats are used in – such as salty air, sea water and harsh weather conditions – meant that normal varnish just was not good enough for that sort of use. Under these conditions, it would only last a short amount of time. So, Spar varnish was developed (source).

The name Spar varnish comes from the spars on ships, also known as rigging, booms and masts that are used to hold the sail. These areas of a boat have little protection against the harsh sea faring conditions. They are also prone to stress, bending and constant flexing in the wind. Normal varnish is not designed to flex in such an environment and – despite being more flexible than polyurethane for instance – very quickly cracks and flakes off.


Spar Varnish.
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What Is Spar Varnish Made of?

Spar varnish typically contains a mixture of four ingredients:

  • Resins,
  • Oils,
  • UV Blockers and
  • Solvents.

We will introduce these constituents in this section. Note that the exact (sub)type of these components as well as the ratios vary among the various manufacturers which can cause small differences between drying times of different products.

Resins

This part of the varnish is designed to create a shiny hard finish to the varnish. Resins are in fact the ingredient that determines the hardness and therefore the resilience and durability of a finish.

However, the more resin is used, the more likely the varnish will lose flexibility and crack. This puts a natural limitation to the maximum share of resins in a varnish mixture as a certain level of flexibility is one of the advantages of varnish types when it is used outside.

Oils

The most common oils used are Linseed oil and Tung Oil. The oil is included in the varnish to penetrate the wood and give a deep level of protection. While raisin creates a protective coat on top of the wooden surface, the oil components strengthen the wood’s resistance to rod and fungus from the inside. This offers a higher quality varnish that will last longer and protect more of the wood.

UV Blockers

UV Blockers are the most important part of Spar varnish. They provide protection against UV radiation and prevent the wood from degrading and from fading color (source). This makes Spar Varnish a good choice not only for boats but also for wood used outside the house.

Solvents

The most common solvents used in Spar varnish are Acetone, mineral spirits or paint thinner. The solvent is in the varnish to act as a carrier for the oils, resins and UV Blockers to be dissolved in. They act as a thinning agent binding all the ingredients together so the varnish can be applied easily in the normal way by using a roller or brush.

How Is Spar Varnish Applied?

Spar varnish can be applied to all types of wood that require long-lasting protection from weather and flexing. However, it is not designed to be applied to wood that is constantly submerged such as the bottoms of wooden boats. For this, a product called Bottom paint should be used that is made especially for underside portions of boat hulls.

To apply Spar varnish, the wood surface must first be clean and free of any loose old paint. Use a sander, a grinder or a sandpaper block to smooth down any rough areas. Then vacuum or blow the dust away with an air hose.

Check the instructions on the Spar varnish can and it will recommend a solvent to use to clean the wood prior to application, although usually, Turpentine works well for this. Some DIYers skip this step if it is the first finish of the wooden workpiece or if it had been finished with oil previously.

Spar varnish can also be applied directly onto new wood or as a coating on top of an epoxy coated surface like those found on new boats and some floating wooden floors or tiles. A little solvent such as Turpentine may be added making the varnish thinner and this also helps it soak into the wood better, hence giving better adhesion (in theory).

Apply it sparsely and two or three thin coats is better than one thick one which will also risk the varnish running. When varnishing outdoors woodwork in the home, 2-4 coats is usually enough, but if treating a boat, for instance, often 8-12 coats may be applied for maximum protection.

The wood surface must be absolutely dry before varnishing otherwise the varnish may not dry properly and take on a white milky appearance. Note also that Spar varnish dries in a couple of hours but it takes two or three weeks to fully cure.

Using Paint Over Spar Varnish

It is a useful tip to know that a coat of Spar varnish can be used on uncoated wood, then paint applied on top of the varnish. The beauty of this is that varnish prevents the paint from soaking into the wood.

If in the future, the paint is required to be removed to give the wood surface a more natural look that shows the grain, the Spar varnish underneath will have stopped the paint’s pigment from soaking into the wood. It can therefore be removed easier using a heat gun. The pigment of the paint will be much easier to sand away, leaving the clean wood and wood pattern.

What Do You Need to Consider When Buying Spar Varnish?

When purchasing Spar varnish, many manufacturers nowadays want to jump on the wagon of success and try to promote their products as Spar varnish products even though they may not include all the ingredients above. Remember a good quality product does not come cheap and if you are tempted by the cheapest brand of varnish, it might not actually be proper Spar marine varnish. It’s always best to check on the can what is actually inside.

Typical Uses of Spar Varnish

What Is Spar Varnish Typically Used for?

  • Can be used on outdoor wood such as garden furniture, decking, exterior doors, wooden facades on buildings and sheds, wooden signs.
  • Can be used on top of old varnish, but rub down and clean with a cleaning product such as Turpentine first.
  • Can be used on top of old paint to seal the wood. Use the same as applying over old varnish and rub down any loose paint, clean with an approved solvent then apply.
  • Can be used as a base coat before painting wood. Then if the paint needs to be stripped off later, the paint pigment will come off easily and will not have soaked into the wood itself.
  • Dyes can be added to the varnish for different coloring effects, however be careful not to add too much dye that will dilute the varnish excessively. However, it is better to use the dye to color the wood as a first step, then once dry, apply a couple of coats of Spar varnish to seal the wood properly.
  • Can be mixed with approximately 25% solvent to get a thinner varnish that will penetrate the wood deeper than undiluted varnish.
  • Ensure coats are dry before applying more. Depending on your location, Spar varnish may take up to 24 hours to dry fully before a second coat can be applied.
  • An oil drying agent such as Japan Drier can be mixed with Spar varnish in order to speed up drying time

When Should Spar Varnish Not Be Used?

  • Wooden items inside the house such as furniture, floorboards, interior doors and stairs that will not be exposed to sunlight and rain. Polyurethane, colorless lacquer and oil finishes are good alternatives, depending on the requirements.
  • Wood that is constantly submersed in water such as the underside of a boat’s hull. For this a special epoxy paint should be used.
  • Paint should not be mixed with Spar varnish. If you want to create a colored wood with varnish effect, apply the paint and then Spar varnish separately.

If the wood surface is not clean and free of debris and grease before applying Spar varnish, you may find the end product becomes streaky. No matter what quality the Spar varnish may be, if it is applied on poorly prepared wood then the resulting varnished finish may be poor. The only option here is to sand back, clean off any debris and wash with solvent then re-apply the coat of varnish.

Varnish vs Spar Varnish – What Exactly Are the Differences?

You may now be asking yourself why you should choose Spar varnish as your preferred finish rather than normal varnish?

Well, Spar varnish is best used for wood that is likely to be exposed to weather, sunlight, humidity or any harsh conditions. This is applicable to exterior wood installations but can also include some woodwork and furniture inside the house, in particular in damp areas. If, for instance, you have decorative woodwork in your bathroom, then Spar varnish is a perfect choice.

However, if you intend to re-finish your antique dining table, a standard furniture varnish would be a better choice.


Spar Varnish Spray.
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Regular Varnish (water-based type).
Check prices of Varnish on Amazon.

This is because Spar varnish is usually optimized for its protective effects rather than its appearance which can be perceived as unnatural in some cases. On the other hand, manufacturers take the look of the treated wood into account when developing regular varnishes. Therefore, they are more appropriate for furniture and other visible objects inside the house.

Conclusion

Both, Spar Varnish and Regular Varnish provide a good level of protection to the wood. Spar varnish is usually more flexible and offers more protection than regular varnish. Regular varnish, on the other hand, retains often a more natural appearance of the wood.

For boats, it is out of question using anything else but Spar varnish for components that are not permanently exposed to water. For DIY uses around the house, Spar varnish will be the right choice for outside wooden installations that require a flexible finish and are heavily exposed to the weather. You can get Spar Varnish in DIY, boating stores or on Amazon.

Indoors, a regular Varnish or alternatives such as oil finishes, polyurethane, lacquer or paint will likely be the better choice.

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