How to Thin and Apply Spar Urethane Correctly?

How to Apply Spar Urethane

Are you looking for additional information on Spar Urethane? How to thin, how to sand and how to apply it correctly?

Well, if this is the case, we’ve got you covered in this article.

Up next, you will find our insights on Spar Urethane along with how to use it for distinct purposes.

Let’s start with the basics first and then see how you can safely use spar urethane to access all its advantages.

What is Spar Urethane?

Spar varnish got this nickname due to the wooden poles you used to find on a marine ship.

The spars on the board of a marine vessel faced a lot of wear and tear due to several environmental factors (like wind, moisture, rain, and harsh sun rays).

So, the spar varnish, that should be applied for protection, needed to be extremely durable and tear-resistant.

As such, sailors created what we call today traditional spar varnish.

This comes with an exciting concentration of resins and solvents to provide a much harder final finishing. And also has the power to offer a smooth finish and an impressive ability to withstand UV damage from the sun. 

As such, spar urethane is a blend of spar varnish that features urethane plastic as the main resin.

And Minwax Helmsman is one leading brand that is known for offering premium quality spar urethane varnish. It is the most frequently used compound for exterior woodwork.

The Applications

Keep in mind that, due to its high resin content, Spar Urethane can lead to yellowish coloring on the wood.

This is why it is not recommended for interior furniture.

Also, it’s not suitable for large exterior surfaces (like decks and siding) where the maintenance of the wood surface would be challenging.

No doubt, it’s a great choice for any kind of woodwork that has been exposed to moisture on a regular basis.

This means you can use it for varnishing the wood doors, windows, trim, bathroom cabinets, kitchen countertops, bar tops, shower seats, outdoor patio furniture, or even your wooden boat.

How to Apply Minwax Spar Urethane?

If you aim to have the best results with Minwax Spar Urethane varnish, you will have to thin it, apply a coat of spar urethane and follow a thorough sanding process between the coats.

Below are the detailed steps you need to follow…

1- Thinning Spar Urethane

The main challenge with polishing spar urethane is that it is an extremely thick compound.

Hence, the first thing you need to know about spar urethane is that it requires dilution to get the right finish.

Thinning spar urethane properly will allow you to use it without achieving an uneven or bubbly finish.

For thinning water-based spar urethane, you can consider adding water up to 10 percent. This will allow you to apply it with ease.

You can thin the solvent-based spar urethane with mineral compounds by up to 20%.

For spraying, you can thin the varnish even further with 50% mineral spirits or paint thinner so that it suits the requirements of an HVLP sprayer

2- Applying Spar Urethane

You can opt for using the brush or an HVLP, but in our opinion, you will manage to achieve a more professional result with a brush.

If using a brush, you will have to apply the mixture with the correct brush, while the movement of your hand must be both sturdy and delicate.

Sponge brushes are the best solution for using spar urethane on woodwork.

Keep in mind that spraying or brushing the spar urethane will require you to add several layers so that everything is covered accordingly.

Another common method of applying spar urethane is wiping. And it can be used even by most inexperienced individuals.

When wiping, you should use clean cotton or a rag so that you have a soft applicator.

Wet the cloth with the thinner and dip it into your diluted finish.

From here on, you will have to apply smooth thin coats and overlap the previous layers so that you ultimately achieve a smooth polished finish.

Keep in mind that with this method, you will spend quite some time on your DIY project, as you will have to repeat the process to achieve a certain level of thickness on the wood. 

Many people opt for using a brush, as it is faster. You will have to clean and dry the wood before applying the substance with your brush.

Make sure the helmsman spar urethane varnish is not too thick, while the ambient temperature should be above 50 degrees Fahrenheit. 

Our expert tip is to brush with the grain of wood, never cross-grain.

This will prevent notable marks on the final result, and we can say from our experience it will offer a nice, crystal-clear finish.

Also, you can add several layers of spar urethane polish on each other, but you should make sure they dry accordingly.

And to make sure there is enough adhesion, it is best to scuff old finishes or existing coats with 150-grit sandpaper.

Afterward, you are free to place the new layer of spar urethane on your wood.

how to apply spar urethane

Why Sanding Between Coats of Spar Urethane is Important?

Indeed, it is rather simple to use and apply spar urethane. But the tricky part is to achieve that perfect finish without any brush strokes.

Hence, one of the most important actions you need to take when applying helmsman spar urethane is sanding between the coats.

This can help you avoid issues such as a bubbly finish and brush stroke marks. 

Also, it will allow the new coat to adhere better, which in turn will improve the end results. 

How to Sand?

For sanding and smoothening the surface, make use of smooth sandpaper. 220-grit sandpaper might just do the job right for you.

Keep in mind that sanding between layers doesn’t mean you need to create a smooth surface.

Instead, it refers to gently removing dust, debris, or other issues to achieve a rough area able to allow a new coat of varnish to adhere. 

After sanding (and before applying the next coat of spar urethane), you will need to make sure there are no paint, wax, grease, or old finishes on your wood.

So, as soon as you clean everything, you can wipe it with a clean cloth, so there is no dust on site.

From this point, you can start adding another thin coat of spar urethane with a sprayer, brush, or cloth. Allow the layer to dry for up to four hours, and follow the sanding process again.

How Many Coats of Spar Urethane Should You Apply?

As seen above, applying more than one coat of varnish is important for getting the best polished and shining surface.

Overall, three coats are more than enough for varnishing exterior wood with Minwax Helmsman spar urethane varnish.

Keep in mind that you should never sand after the last coat of spar urethane. Also, it is best to allow the surface to dry for at least one day before using it.

How Long Does it Take for Spar Urethane to Dry?

Drying time for spar urethane is typically based on environmental conditions and other factors.

Under normal conditions (i.e., 70°F and 50% relative humidity), it may dry to touch in about 2 hours.

However, you can re-coat the surface after 6-8 hours.

The surface gets ready for usage after 24 hours.

But it’s recommended that you give it more time (of up to 3 days) before you start using your polished surface.

Remember – you will need to dry the surface for a longer time at cooler temperatures or if the humidity is high.

Spar Varnish Vs. Spar Urethane: What is the Difference?

Spar varnish and spar urethane are two compounds that may seem to be the same but are actually very different.

The basic difference between these two is spar varnish may get chipped or chalked off over time, but spar urethane does not.

Spar urethane will instead make a shiny hard surface and maintain the shine for a long without getting affected.

Spar Varnish

Spar varnish is usually manufactured using natural oils such as linseed, soya, or tung.

These oils act like base oil and are integrated with solvents or resins to create a protective formulation for wood.

When applied to the surface and dried, it creates a shiny surface.

Spar varnish, in general, protects the wood from getting damaged or discolored due to water, heat, UV rays, stain, and solvents.

Depending on the climate conditions, the drying time between the coats of spar varnish may go from 24 to 36 hours.

Although it provides a protective scratch resistance coating, the disadvantage of using spar varnish is it chips, chalk, or flakes off over time.

So, it needs to be reapplied on the surface periodically to create a protective layer.

Spar Urethane

Spar urethane, on the other hand, is a synthetic product that is usually made of water-based or oil-based products.

Water-based spar urethane dries very soon and can be cleaned easily.

While the application of oil-based spar urethane needs more time. You may need to wait between the application of coats for a few hours.

Plus, you will need to apply it slowly and in a thin layer to avoid dripping.

Depending on the climatic conditions, the drying time of spar urethane is comparatively short and may go from 30 minutes to 120 minutes.

Related FAQs

How long does spar urethane last?

Oil-based varnishes and helmsman spar urethane doesn’t go bad soon and can last up to 10 to 15 years if applied correctly.

How soon the shine may fade will also depend on the kind of finish you have applied (like satin, gloss, etc.).

Water-based spar urethane, however, has lesser life and may last for about 3-5 years, depending on the conditions.

Why does spar urethane turn white?

Spar urethane may often turn white if there is any excess moisture present in the wood.

This is generally the case when it’s varnished on wooden boats. During the color of the spar urethane changes, it’s not a sign of damage.

Can you apply spar urethane over the stain?

Absolutely, you can apply Helmsman® Spar Urethane polish over the stain.

But there may be a slight ambering (yellowish to brownish translucent) layer experienced, especially when it’s applied over light-colored stains on the wood surfaces.

Should you remove old spar urethane before re-coating?

Yes, if there is a heavy coating applied on a wood surface, it’s best to remove it before you re-coat.

You can use a bit of chemical stripper and then scrape the spar urethane off the surface using a plastic scraper.

Avoid using a metal scraper, as it can damage the surface. Once done, clean the surface with a cloth damped in mild soapy water to remove the chemical residue.

The bottom line

Applying Spar urethane is one of the best ways to offer good protection to your wood surfaces (both indoors as well as outdoors).

The oils contained will allow the finish to contract and expand without getting cracked, chipped, or bubbled out.

Not only does this enhance the natural beauty of the wood, but it also prevents the wood damage from various elements for a long.


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