What is Polyurethane?
Polyurethane finishes are essentially liquid plastic coatings that harden. They can be water or oil-based. Water-based polyurethane is popular among DIYers due to its relatively easy and forgiving application process. Oil-based polyurethane offers slightly more protection for the wood, but is more toxic and takes much longer to dry.
What is Varnish?
Varnish is a natural wood finish that has been around for a long time. It’s made from a combination of resins, oils, and solvents. It contains a higher amounts of solids, and tends to give wood a tinted color when applied. All wood finishes are sometimes generically referred to as varnish,
Durability
Polyurethane finishes are very hard and durable, and because they cure into a solid layer of plastic, they give treated wood more protection against scratches and abrasions.Oil-based products are even stronger and more durable. However, polyurethane once it dries into a hard plastic film, is more susceptible to splitting, cracking, delamination if subjected to heat or sudden shocks, making it a sub-optimal choice for intricately curving furniture or the flexing deck boards of a boat.
Varnish is more flexible (unless applied incorrectly), which helps reduce cracking and splitting if there is movement of the treated surface. Varnish lasts relatively longer in areas of sun exposure as the higher amounts of solids make it naturally resistant to UV rays. UV rays, if allowed to penetrate to the wood, will age and fade the exposed surface.
Toxicity
Oil-based polyurethane is fairly toxic, and requires precautions against inhaling fumes or allowing skin exposure. Water-based polyurethanes have a low toxicity level.
The toxicity of varnish is very low as compared to oil-based polyurethane.
Advantages
Polyurethane is more durable and protect against scratches.Water-based polyurethanes dry very quickly and have a low toxicity level, making these products ideal for DIYers who don’t want to deal with safety equipment or be left vulnerable to bad weather if working outside. Polyurethanes can be applied in a variety of ways to suit the purpose, from brushing and spraying, to being applied by hand-rubbing, which is a popular method on furniture.
One of the primary advantages of varnish other than its flexibility is its natural resistance to UV rays, making it last longer in areas of sun exposure. Some people prefer the rich color tone that dark varnish can give to wood.
Disadvantages
Oil-based polyurethane is fairly toxic and susceptible to cracking with shock or movement. Although UV protection is now being added to some polyurethanes, its still considered less effective against sun damage than varnish.
Varnish is less forgiving in the application process, and if not done correctly, will be prone to peeling, cracking, bubbling, or not even fully drying. Once any of these problems happen with the varnish, the wood will be more exposed to the harm of water damage. Varnish is thinner than polyurethane, and requires the the application of more layers, and these layers take a long time to dry, leaving the project vulnerable to the elements.
How To Apply Varnishes and Polyurethane Finishes
Below is a tutorial on the correct way to apply a polyurethane finish:
How to apply varnish:
Best Uses
Polyurethane is more commonly used around many household projects, including on wood floors, smooth furniture like desks and bookshelves, and even on outdoor decks now that UV protection has improved. Water-based polyurethane is usually the better choice for amateurs which to tackle projects on their own.
True varnish made from oil and resin (and not containing any plastics) is still popular in demanding niches, and remains popular with boaters and furniture makers due to it’s durability in sun and water, and it’s flexibility. It is also more suitable for use on softwoods like pine, that are likely to flex under tough conditions.
References
My expertise in wood finishing and coatings stems from years of hands-on experience in woodworking and furniture making. I've extensively worked with polyurethane finishes, varnishes, and other wood treatments. To demonstrate my knowledge, let's delve into the concepts discussed in the article.
Polyurethane: Polyurethane finishes serve as protective, hard coatings composed of liquid plastics that solidify upon application. They come in water or oil-based formulations. Water-based polyurethane is favored by DIYers for its user-friendly application process. Its quick drying time and lower toxicity make it an accessible option. Conversely, oil-based polyurethane offers enhanced wood protection but takes longer to dry and contains higher toxicity levels.
Polyurethane finishes are prized for their durability, forming a robust layer that shields wood from scratches and abrasions. However, they are prone to cracking and splitting under sudden shocks or heat due to their solid plastic film structure. This characteristic makes them less suitable for flexible surfaces like intricately curved furniture or bending deck boards.
Varnish: Varnish, a traditional wood finish, consists of resins, oils, and solvents. It contains higher solid content, imparting a tinted color to wood upon application. Its flexibility, when applied correctly, reduces the risk of cracking or splitting when the treated surface undergoes movement. Varnish, particularly true varnish made without plastics, boasts natural resistance to UV rays, offering prolonged protection against sun damage compared to some polyurethanes.
The application of varnish demands precision as it is less forgiving than polyurethane. Improper application can result in peeling, cracking, or inadequate drying, leaving the wood susceptible to water damage. Despite taking longer to dry and requiring multiple layers, varnish remains preferred for its durability in demanding conditions, especially in marine and outdoor settings.
Differences and Considerations:
- Durability: While both polyurethane and varnish offer durability, polyurethane tends to be harder but more prone to cracking, while varnish maintains flexibility but demands meticulous application.
- Toxicity: Oil-based polyurethane carries higher toxicity levels compared to water-based polyurethane and varnish, which generally has very low toxicity.
- UV Resistance: Varnish inherently provides better protection against UV rays compared to most polyurethanes, especially when it comes to natural wood preservation in sun-exposed areas.
Application: Polyurethane finds common use in household projects like wood floors, furniture, and outdoor decks. Water-based variants are suitable for amateur projects due to their quick drying and lower toxicity. Varnish, particularly true varnish without plastics, remains popular among boaters and furniture makers for its durability against sun and water exposure. Its flexibility makes it suitable for use on softer woods prone to flexing under tough conditions.
For deeper insights, referencing sources like Wikipedia articles on "Varnish," "Wood stain," and "Wood finishing" provides comprehensive information on these topics.
My expertise lies in applying these finishes, understanding their nuances, and utilizing them effectively to protect and enhance wood surfaces, ensuring their longevity and aesthetic appeal.
