As a woodworker, you will come across situations regularly where you have to apply finish to wooden surfaces. The process of coating wood with a protective or decorative layer is known as “finishing.” We get different types of finish, depending on the type of surface that we are finishing.
The various types of finish are varnish, polyurethane, lacquer, and shellac. These products are interchanged sometimes, and frequently mixed with each other. We may also mix solvents like water, oil, alcohol or spirit to achieve the best consistency.
In this post, we discuss varnish and polyurethane and discuss where each one is best suited. We also highlight the way each of these two finishes should be applied to a wood surface. On reading this post, you should be confident about where and when to use either varnish or polyurethane. So, let’s get started!
Varnish Vs Polyurethane
Whether you have outdoor or indoor furniture, hardwood floors or outdoor decking, you need to apply a suitable finish if you want your woodwork to remain fresh-looking and have a long life. If you don’t protect your wooden surfaces, you will find that you need to refinish it frequently. Some surfaces require a polyurethane surface and others require varnish
When you have to apply a finish to a wooden surface, you can get the best results if you use the right type of finish and apply it correctly. Polyurethane and varnish are two types of finish that play a prominent role in the field of woodworking.
Even though we tend to mention both these coatings in the same breath, each one is distinctively different from the other. Let us consider the finer details of each type of finish:
Polyurethane
We can best describe polyurethane as liquid plastic. Hence, when it dries, it has the consistency of plastic. The two varieties of polyurethane are oil-based and water-based, and both come in satin and glossy finishes. Water-based polyurethane emits less odor and is less toxic and dries quickly. However, it has a low resistance to heat and chemicals. Water-based polyurethane is more expensive than the oil-based variety. An excellent example of water-based polyurethane is Minwax Polycrylic.
Polyurethane Uses
When you have to apply a finish to hardwood floors, polyurethane is best-suited for the purpose. You get two options with polyurethane – oil-based polyurethane and water-based polyurethane. If you want to avoid applying multiple coats and would like to get a rich color, oil-based polyurethane will be suitable for your purposes. If you would like the natural color of the wood to come through the finish, then you can use water-based polyurethane.
Water-based polyurethane is more expensive than the oil-based variety, but then it depends on your requirement. Another advantage of using water-based polyurethane is that it is low on odor and toxicity levels are also lower. It will also dry faster. You can get satisfactory results by using water-based polyurethane on bookcases, desks, side tables and picture frames or any surfaces that will not be subjected to extreme conditions.
How to Apply Polyurethane
Deciding the process of applying polyurethane depends on the base that it contains. Here are some tips about using the two basic types of polyurethane:
You can get two types of water-based polyurethane – the conventional type and a hybrid variety that we refer to as water-based, oil-modified polyurethane. Apply both these types with a natural fine-bristle brush, a rag, or foam roller. You can also apply it using a spraying device.
To make each coat adhere better, you can rub some fine steel wool across the surface of the wood. The more the water content, the thinner the polyurethane will become, and you will have to apply more coats for a fine finish.
Regarding oil-based polyurethane, you can apply it using similar methods, but keep in mind that your work area should be well-ventilated until the coating has dried. Using a respirator is also mandatory to protect yourself from volatile fumes.
Ensure that you wear disposable gloves before you start handling the polyurethane finish. Also, take careful note of the manufacturer’s directions before you commence the job to get the best results.
You can apply both types of polyurethane finish to latex or acrylic paint. You may get a yellow or amber tint if you use oil-based polyurethane, especially on light colors.
Another good alternative available is polyurethane spray which comes in a spray can. You will find this very convenient for big projects and for performing touch-up jobs.
Varnish
We refer loosely to any topcoat on wooden surfaces as “varnish.” By applying varnish to wood, you impart a certain level of protection against the elements and insect attack. Varnish also adds to the luster of the wooden surface, enhancing its appearance. On curing, varnish forms a thin, glossy film on the wood with a yellowish or amber tint, just like with oil-based polyurethane.
You may not find varnish used so much on indoor surfaces the way we use polyurethane like on hardwood floors. We use water and oil in combination with plastic resins to make polyurethane. Varnish, however, contains mostly alkyd resins, oils and solvents. Varnish also offers higher resistance to ultraviolet (UV) radiation than polyurethane because of the high levels of solids that it contains. It is another reason why varnish is a suitable choice for finishing outdoor wooden surfaces.
With varnish, you get a sort of colored tint and consequently, you have to apply more coats on the surface of the wood than polyurethane.
Varnish Uses
Varnish contains a high solid content and has a fair degree of water resistance. Hence, you will find varnish to be suitable for finishing wooden surfaces of outdoor items like boats, deck chairs and picnic tables. Varnish has low toxicity which makes it food-safe to use on wooden kitchenware like spoons, bowls and spatulas.
You can also get a class of varnish called “spar” which works well in a marine environment, and it also offers a higher level of UV protection. This class of varnish is flexible which makes it popular with DIY woodworkers who use softer woods such as pine and Douglas fir which bend easily.
How to Apply Varnish
Unlike polyurethane, you need considerable skill to apply varnish. If you don’t apply it properly, it can crack and peel and even form bubbles. If such defects occur in your coat of varnish, then the wood becomes prone to damage from water and the environment.
To apply varnish properly, you should use a natural-bristle brush and mix the correct ratio of solvents and oils to achieve the required consistency. Refer to the manufacturer’s directions to ensure proper mixing and application.
Allow sufficient time for the varnish to dry between coats, and rub the surface with fine-grade sandpaper or steel wool. It would be best if you left the final coat to dry for at least six hours for it to dry completely, and 24 hours for the film to get entirely cured.
After reading the above information, you can see the table below that summarizes the differences between polyurethane and varnish:
| Parameter | Polyurethane | Varnish |
| Available form | Oil or water-based | A mixture of oils, resins and solvents |
| General Properties | Hard and resists abrasion | Flexible but scratches easily |
| Ultraviolet (UV) protection | Low | High |
| Drying and curing time | 2-3 hours | 6 hours or more |
| Preferred method of application | Brush, wipe or spray | Best results by brush |
| Flexibility | Brittle surface | Flexible surface |
| Toxicity | Medium to high | Low |
| Ease of Application | Easy to apply | Needs some skill |
| Usage | Indoor wooden furniture | Outdoor wooden furniture |
Conclusion
We use polyurethane for indoor wooden items like desks, shelves, tabletops, chairs and sideboards. Varnish finds use in any woodworking outdoor projects exposed to the elements. Hardwood floors are of course best if coated with polyurethane.
Both polyurethane and varnish are equally useful, but you need to know where and how to use them. With this information, you should be more knowledgeable about how best you can use either of these two types of finish. Now, you can work confidently with the reassurance that you can apply the best final touches to your wooden surfaces.
