Spar Urethane over Polyurethane – How to Apply Correctly?

Woodcrafts / By

If you have a wooden door or other wooden accents to the outside of your home, there are various ways to protect them from the elements. The most popular substances to use on a door are Polyurethane and Spar Urethane. They are often mentioned separately, but can you layer them for maximum protection and durability?

Yes, you can apply Spar Urethane over Polyurethane so long as the base layer of Poly has completely dried. The polyurethane acts as a base layer of protection, and the Spar Urethane will provide a shield from sunlight and day-to-day wear. Both will give you added protection to the wood, but they are layered in a specific way and accomplish slightly different ends when it comes to protecting your wood door.

Keep reading to learn more about the differences between Spar Urethane and Polyurethane and how to safely layer them to get the best results.

Isn’t Paint Enough?

Both Spar Urethane and Polyurethane are types of varnish that add protection to the wood. Paint does add a layer of protection, but not every homeowner wants their wood painted over. Some prefer the look of natural wood and appreciate the beauty of swirling grains and knots.

Wood comes in diverse species and varieties, and each wood has its unique colors, warmth, coolness, and character. Preserving the natural beauty of wood and allowing it to shine through on a natural wood door is a great way to welcome people into your home and take greater pride in your investment.

Varnish adds a barrier between your natural wood door and the rest of the world. So let’s take a look at the differences between Spar and Polyurethane, and see how they can work together.

What is Spar Urethane?

Spar Urethane is a clear, protective finish that works to protect the interior and exterior wood from weather damage, such as sunlight, rain, or extreme temperatures. Originally formulated for use on boats, Spar Urethane is superlative at protecting wood from the elements and is meant to last for years.

It’s thick and looks polished once dried, and also protects against discoloration (wood can often fade to grey in the sun over time). Special oils within Spar Urethane allow the wood to contract and expand safely as the temperatures change year-round, and help keep the wood from drying out, too.

Feel free to use Spar over natural wood as well as over stains, as it dries clear, and doesn’t discolor the look of the underlying wood grain. Spar Urethane cleans up easily with warm water and dries quickly.

What is Polyurethane?

Polyurethane is perhaps the most common door sealant and clear varnish. It comes in oil, water, and “water-based oil-modified” formats, and gives your door a nice, simple layer of protection without the added color of a stain.

Oil-based polyurethanes tend to give wood a very subtle warm glow, which will adjust to the natural color of your wooden door. They clean up with mineral spirits and dry slower than water-based polyurethanes.

Water-based polyurethanes are completely clear. They clean up with soap and water and dry a bit faster, but will need more coats than an oil-based varnish.

Water-based oil-modified polyurethane seems semi-opaque in the can but will dry to a tough and lightly warm finish, very much like an oil-based sealant.

Polyurethane is a simple and sturdy sealant and works nicely to keep your door protected, without masking the grain’s natural color or patterns. It’s a good base layer to keep in mind next time you’re replacing wood fixtures around your home.

How Do You Apply Polyurethane?

Polyurethane is easy to apply but does take some patience and a bit of care. It’s a thinner varnish that is easy to apply and doesn’t require any nuanced application methods.

You will want to make sure that your door is clean, though. The first step to applying polyurethane is to carefully clean the door and the surrounding area of any dust, debris, or pollen that might get in the way, or damage the sealant job. Make sure your door is completely grime-free and impurity free before starting.

You’ll need to apply the first layer of polyurethane and allow it to completely dry, for about 24 hours. Once it’s dried, go ahead and give your door a light sanding down, to smooth out any blemishes in the dried varnish.

Next, apply the second layer, allowing this to dry over 24 hours, as well. Give each dried varnish job a light sanding before applying the next layer for the smoothest possible results. The Polyurethane can be applied with any small brush and should be lightly visible as you work.

Why Would You Want to Layer Spar over Poly?

Basically, the more protected your project is, the longer it will last and the nicer it will look, too. Some consumers prefer the high gloss of Spar Urethane and appreciate its greater strength and durability. Using Poly as a base and Spar as a top coat can give your exterior door several added layers of protection.

Spar Urethane tends to give off a bit of brightness and glow when applied and this can look very nice on a clear layer of poly. The more layers you use the better protected the natural color, quality, moisture, and integrity of your wooden door will be.

Spar Urethane also lends extra UV protection that Polyurethane simply isn’t formulated to do. So by applying both, you can get a gentle glow and a beautiful, natural wood look without compromising on protecting your wooden door from harsh weather, and sunlight.

If your door sees a lot of use and handling, has to weather flying Fed Ex packages, or is exposed to extreme summers and extreme winters, layering Poly and Spar is a smart way to help your door survive all this, and still look great. After all, you’ve invested a lot in your wooden door, and you want it to last as long as possible.

How to Successfully Apply Spar over Poly

So how do you apply Spar over Poly? You’ll want to lightly sand down your dried Poly to allow the Spar to adhere to the dried Poly varnish nicely. If you fail to sand down, the layer of Spar will often peel, and the results will have to be corrected.

The simplest thing to do is rub down your dried Polyurethane coat with a 220-grit hand sheet, then go over it with your layer of Spar. You can do this safely so long as the Poly has hardened and cured fully.

You’re going to want to multi-layer the Spar Urethane, too, for the best results. The thicker your Spar, the better protected your wooden project, and the underlying layers of Poly will be from harsh sunlight, bad weather, extreme temperatures, impact, and drying out. You’re not just protecting the wood, but the armor, too.

It’s really that simple. Just dry, lightly sand, and layer. Let’s go over a few ways to apply Spar Urethane in a way that’ll make it look beautiful, and go on easier, too.

Tips for Applying Spar

One thing you will notice about Spar Urethane is that it is incredibly thick, nearly similar to corn syrup. As a result, brush strokes can be more clearly visible in a dried coat of Spar Urethane, and this can drive some folks nuts.

To cut back on this, try cutting the Spar Urethane with a 10-20% mineral spirits mix. This should thin out the Spar just enough to make it more workable and allow it to go on smoother. You may need to layer it a bit more to make up for the slightly thinner consistency, but your brushstrokes should be less noticeable once you’re done.

Wiping is another, simpler method for applying Spar Urethane.  Thin your Spar by cutting it with a 50% mineral spirits ratio. This will make it very thin, and very workable. Apply gently with a clean, cotton cloth, overlapping each pass as you go.

Wiping the Spar on is a bit slower, but works great to eliminate brushstrokes, and give a very smooth, natural appearance. You’ll have to double the number of coats with this method, but Spar tends to dry faster than Poly, so it shouldn’t take you quite as long.

Conclusion

Yes, you can safely apply Spar Urethane over Polyurethane. Poly is a very good basic varnish, whereas Spar is a superior UV shield. Both are clear varnishes meant to protect the wood from weather damage, and they work great on exterior elements.

Make sure that your Polyurethane has completely dried, then gently sand it down with 220-grit sandpaper, and apply your Spar Urethane layers. Be sure to layer the Spar on thick for maximum UV protection and weather resistance.

While Poly is thinner and takes longer to dry (24 hours between coats), Spar dries much faster and will need several layers. Both can add beauty, protection, and longevity to any woodwork exposed to the elements.

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