Spar Urethane over Shellac – How Will This Work?

Woodcrafts / By

You have reached the final treatment of your wooden piece and you are wondering how to protect it optimally. The purpose of use, as well as the accommodation, largely depends on the final protection of the wood.

There are differences in the final protection depending on where the object is located. If it is exposed to external influences, especially if it is located near the sea or freshwater, the protection is different than if it is located indoors.

One of the common coatings is Shellac, a well-known finish for all DIYers and woodworkers. In this article, we will deal with the question of whether spar urethane applied to shellac, would be a good protection and whether this combination is even possible.

To be able to answer this question correctly, we will first briefly explain each of the mentioned coatings, as well as their purpose and properties. We are going to start with Shellac.

What is shellac

Shellac is a natural finish and it is known for thousands of years. It is made from the female lac bug secretion that is dissolved in ethanol, and it naturally contains wax. This coat enhances the natural color of the wood and protects it by creating a protective film on the surface of wooden objects.

Shellac evaporates quickly, which means that it dries quickly and that it can be applied in several layers during the day. The colors of this finish range from clear to garnet and amber. Over time shellac darkens and develops a patina.

How to apply shellac

It is easy to apply shellac and it can be done by sprayer or simply with a brush or rug.

It is best to apply it generously to the surface in long and smooth strokes following the grain of the wood. When the first coat is dried completely you need to sand it with a sandpaper range of 320 to 400. After that, you can apply the next coat.

What is shellac best for

One of the most important features of this coating is that it is soluble in alcohol and that means it is easy to remove it from wooden surfaces with alcohol. That is why it is not the best idea to put it on the dining table, the bar, or any other piece of furniture where alcohol is served.

The disadvantage of shellac is that it is easily damaged by water or alcohol, but it is a great finish for jewelry boxes, drawers, wooden decorations, closets, etс.

If you want a long-lasting finish, a transparent sealant must be applied to the shellac. This is why woodworkers and DIYers are trying to find the best coating to protect the shellac.

What is spar urethane?

Unlike shellac, spar urethane is a synthetic product. It is characterized by high resistance to water and external conditions, so it is most often used to protect ships and other outdoor wooden products. This finish is much more durable than classic varnish and lasts much longer.

Due to the higher proportion of oil, it contains, it is also more resistant to the wood expanding and contracts under the influence of temperatures and weather conditions. Because of all the mentioned characteristics, it has become very popular among DIYers as well.

Spar urethane types

This coating is available as oil-based and water-based spar urethane.

Oil-based is best to apply on harder-wearing interiors and exterior products, while water-based dry faster and is good for standard-wearing furniture. Water-based dries quickly, while oil-based gives a much more durable finish. Water-based spar urethane is milky until it dries completely.

The look of spar urethane

It comes in various finishes, from satin to semi-gloss and gloss. Urethane protects the wood surface from the sun’s UV rays, and because of that original color and wood grain don’t fade, although its high resin content gives it a yellowish tint.

How to apply Spar Urethane

Spar urethane can be applied to wood in a similar way as standard coatings, by using a brush or a roller. You can also use a spray gun but in that case, you need to thin down spar urethane with 20 to 30 % with mineral spirits. You can also apply it using the wiping method.

After getting acquainted with both coatings and their properties, we will deal with the question from the title of the article. Can you put spar urethane over shellac and how will that work?

Spar urethane over shellac?

When you take a close look at the purchased can of spar urethane, you will see that it says not to be used over shellac. The reason for this is that urethane does not adhere well, and even does not stick well to itself. For a long time, you could hear never to put urethane, or polyurethane, over shellac.

This was a recommendation you could hear from most of the woodworkers, from the newspapers to various shows and magazines. This is the reason why many people have never tried this combination. But that doesn’t necessarily mean that it is not possible or even that is completely wrong.

Recently, there are more and more cases where woodworkers have decided to use this combination of coatings and according to their experiences, no dramatic situation or terrible damage happened. This means that the answer to the question is that it is possible to put spar urethane over shellac.

With this procedure, you will get a beautiful finish look, but also permanent and resistant protection of the wood.

Waxed or dewaxed shellac?

Some sources say that if you want to use this combination of sealer and finish, you must pay attention to what shellac you are using. Some say that de-waxed shellac is the right one to use.

If you want de-waxed shellac you may need to produce it by yourself. You can also buy it, but it is not a common product and is hard to find. Shellac naturally contains the wax, and you can de-waxed it by straining it through, for example, coffee filters or you can place shellac in the sun, which will heat it and remove the wax quickly. Then you can drain the transparent shellac from the top.

Others say that it is not necessary to do that and that you can use both types of shellac. If you want the piece you are processing to have a beautiful appearance and durability, you must pay a lot of attention to the correct application of these two products.

But of course, everything starts with the correct preparation before applying the finish.

Preparation of the wood surface

When you have applied the shellac in the layers you wanted, it is time to adequately prepare the surface for coating with spar urethane. Although when finishing with spar urethane, many people emphasize its insufficiently adhesive properties, most often it is due to improper preparation.

Before applying spar urethane, the surface must be sanded, and that procedure must be done after every new coat. Pay special attention to whether the surface is sufficiently dust-free and clean. This way, we improve the ability of the coat to stick better.

If you skip the above steps when applying spar varnish over regular shellac, you will likely notice how your urethane spar “slides” over the top of the wax-based shellac.

Best way to apply spar urethane over shellac

Spar Urethane, as we mentioned, can be applied in all the usual ways; with the help of a quality brush with natural fibers or a quality roller. But many woodworkers believe that the wiping method gives the best finish.

This method is considered by many to be the easiest, so it is great for beginners. This method eliminates brushes and rollers and replaces them with a clean cotton cloth. When applying this method, it is necessary to dilute spar urethane with 50% mineral alcohol, thinner, or petroleum.

Wet a cotton cloth with the resulting mixture, then wipe the surface of the wood in smooth and thin layers. With each stroke, fold the previous stroke in half.

The advantage of this method is that the finish will be free of bubbles and visible brush strokes. The disadvantage of this method, if we can even call it that, is that twice as many layers must be applied to achieve the desired film thickness.

But, on the other hand, due to the thinned material, the drying time is also reduced.

Final thoughts

Even though many believe that spar urethane can’t go over shellac it is not the complete truth. Many woodworkers have proven that it is possible and that with a combination of these two finishes we can get a beautiful yet durable finish.

Be careful how you treat the surface and don’t forget to sand after each layer. If you do everything in the specified order, then you will enjoy the beautiful appearance of your wood for a long time.

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