Polyurethane is a water- or oil-based plastic resin used for coating wood or as a wood finish. Varnish is an older type of finish made from resins, oils, and solvents, but very often, the term "varnish" is misused as a generic name for all types of wood finishing.
This comparison talks about polyurethane only in the context of wood-finishing and coating, not in the context of manufacturing high-resilience foam products as it is usually understood.
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,
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.
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.
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.
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:
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.