One of the essential stages of woodworking is applying a finish to the wood. Even the most skillfully-executed woodworking project will appear incomplete if it doesn’t have a proper finish. By “finish,” we refer to applying a coating to the surface of the wood to enhance its appearance and add a protective layer to it.
We get several substances for applying a finish to wooden surfaces. We have discussed different types of wood finish in another of our posts. Here, we highlight two popular wood finishes, shellac, and polyurethane varnish. We discuss the similarities and differences between the two and how and where we can use them.
Shellac vs. Polyurethane
While shellac is a natural extract, polyurethane is a synthetically produced product. Shellac comes from the female lac bug (Laccifer lacca), and it is a natural resin. Besides serving the purpose of applying a wood finish with shellac, we also use it in various industries like in the pharma, dentist, and cosmetic fields.
We get two types of polyurethane varnish, water-based and oil-based. It comes in two different types of finish, satin, and gloss. Let’s look at the features of each kind of finish more closely:
What is Shellac?
As we mentioned above, shellac is a natural product that we produce using resin from the female lac bug. We mix it with a solvent like alcohol to apply the product to wood surfaces. Once dried and hardened, shellac forms a safe and durable coating on the wood.
Shellac also imparts a warm, amber color to the surface of the wood. A downside is that white rings may form if you put a warm or wet mug or bowl on a shellac surface. Hence, shellac isn’t the ideal choice for kitchen or dining room furniture.
Uses of Shellac
Let us consider where we can best use shellac. We use shellac on fine furniture items, which will bring out a fine polish once coated with shellac. Sometimes, you will see shellac coatings on non-wood articles as well. You can use shellac on all wooden surfaces that are not likely to be exposed to heat or moisture. You can apply shellac with a brush or a cotton rag.
What is Polyurethane Varnish?
We could best define polyurethane varnish as liquid plastic. You can get both water-based and oil-based polyurethane, and the finish can range from satin to glossy. Water-based polyurethane is the more-favored variety because it has low toxicity and not much odor. You can quickly get a clear finish with water-based polyurethane. However, water-based polyurethane may not be adequately resistant to heat and moisture.
When it comes to oil-based polyurethane, you will find slightly higher durability than water-based polyurethane. The drying and curing time is more for oil-based polyurethane than the water-based type, and it can vary from one manufacturer to another. Hence, it would help if you referred to the manufacturer’s recommendations regarding the drying and curing time of a particular brand.
Oil-based polyurethane has a better tolerance to heat and moisture so that you can use this finish on kitchen and dining room furniture. Like shellac, you can apply polyurethane with a natural-bristle brush or a rag.
Where you get a clear finish with water-based polyurethane varnish, the oil-based variety tends to impart a yellowish hue to the wood. Therefore, you need to choose the type of polyurethane varnish to use depending on your requirements.
You can get polyurethane in spray cans, which makes the process quicker and easier. While spraying on the finish is convenient, many woodworkers prefer the wipe-on variety of polyurethane. It creates a hand-rubbed finish. You can get excellent results with both methods of application.
Uses of Polyurethane
Polyurethane varnish is best used on wall-hanging items like picture frames, bookcases, and indoor furniture that won’t have to face the elements. You can also use it on wooden flooring. Polyurethane varnish also plays a prominent role in finishing wooden surfaces of cabinets.
If you are using the oil-based type of varnish, you can get the advantage of resistance to water and heat.
Comparison between Shellac and Polyurethane
Given in the table below are the primary differences between shellac and polyurethane varnish:
|Water Resistance||Low||More than Shellac|
|Heat resistance||Low||More than Shellac|
|Color||Yellowish tint||Yellow to crystal-clear|
|Drying time||1 hour||30 minutes to 2 hours|
|Durability||Less durable||More than Shellac|
The Importance of Applicators
The best practice is to use natural brushes (also called China brushes) for applying an oil-based finish to wooden surfaces. You should use synthetic (nylon) brushes to use with latex, acrylic, or water-based finish. Also, you can use rollers and rags where you get good results. Always use a test piece for trying out your applicator and finish before starting the job, to see how the final result is likely to look.
While applying a top coat (the last and final coat) ensures a dust-free environment for a blemish-free finish. On entering the room where you will apply the finish, be still for some time and allow no air movement in the room, preferably not even the air from a fan. It will enable the dust to settle. In extreme cases, you may need to use some form of air purifier.
While you need to apply multiple coats for a durable and attractive finish, avoid applying too many coats. Four or five coats are usually the maximum number of coats that should create a complete finish. Maintain the practice of “keying” when you rub-down the coating once dry with fine-grit sandpaper or steel wool. After keying, make sure that you wipe off the residual powder with a brush and dry cloth before adding the next coat.
Both these types of wood varnish serve a vast number of purposes in woodworking. If you want to be environmental-friendly and look for a more natural look, then shellac is a suitable choice. If, however, you need a modernistic look to your wood and you want a clear finish, then polyurethane varnish will satisfy your requirements.
Further, the oil-based variety of polyurethane varnish offers more durability in terms of moisture and heat resistance. You can choose the type of finish you want to use according to the requirements of your projects. You can look at our table for quick reference. If you select your finish carefully and apply it correctly, you will impart a professional look to all the surfaces in your woodworking projects.