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In this article, learn more about varnish and polyurethane, including their similarities and differences. We’ll also share the major differentiating factor between varnish and polyurethane, when to use each kind of finish, and which is best overall.
- What Is Varnish?
- What is Polyurethane?
- Varnish vs Polyurethane
- Major Differentiating Factor
- When to Use Varnish
- When to Use Polyurethane
- Which Is Better, Varnish or Polyurethane?
What Is Varnish?
From a purely technical standpoint, a varnish is any finish that contains a resin and dries to a clear finish. Varnishes are used to protect and beautify wood. They contain a blend of resin, drying oils and driers, and evaporative solvents.
Catalyzed varnishes include a chemical catalyst that initiates a curing process. Fully-cured varnish is harder and more durable than non-curing versions.
Varnishes are a significant source of VOCs. These compounds are present not only during application, but continue to be released even after the finish dries.
It is important to protect your lungs from VOCs by using a respirator-style mask and applying varnish only in well-ventilated areas.
Varnish is usually thinned significantly before being used to seal wood. Higher concentrations are used to build the finish, and the final coat is usually full-strength varnish.
Thinners can also be used on an as-needed basis, to improve spreadability and control dry time.
For a super-smooth finish, an HVLP sprayer can be used to apply properly thinned varnish. Varnish can also be brushed on. A significantly thinned formulation of varnish can be wiped on with a rag.
Applying spar varnish, which is typically for more intense environments like water, is a little bit more complicated but very similar in process.
The properties and qualities of varnish are dependent on the ingredients used. The type of resin and the amount and type of oil used determine the color, hardness, and application of varnish.
- Acrylic or alkyd resin varnishes are crystal clear, and will not yellow with age or exposure to UV light. They are usually intended for interior applications.
- Nitrocellulose varnishes include a phenolic resin which cures to an extra-hard finish and is perfect for exterior use. They also have a natural amber glow that enhances the look of medium and darker-colored woods.
- Urethane-based varnishes offer the toughest, most durable protection. Polyurethane, one of the most popular and well-known finishes available, is made from multiple kinds of urethane. Oil-based polyurethane is therefore, at least technically, a type of varnish.
What is Polyurethane?
Polyurethane is an extremely durable and protective clear coat, usually applied to wood surfaces. It can be applied directly to wood or as a top coat.
It’s exceptional hardness comes from the polymers used in formulation of polyurethane, and a chemical reaction called ‘curing’ that takes place as the finish is exposed to oxygen.
Oil-based polyurethanes are the most durable. However, they also take a long time to dry and cure. It is unlikely that you can apply two coats of oil-based polyurethane in a single day. It takes several weeks for oil-based polyurethane to complete the two-phase hardening process.
Water based polyurethane is less durable than oil-based products. The advantages to water polyurethane are that it emits minimal to no VOCs, dries within a few hours, and cures within a few days. The odor is considerably less offensive and dissipates more quickly than that of oil-based polyurethane.
While water polyurethane does not emit high levels of VOCs, there are isocyanates present which can cause problems if you become sensitized. Always wear a respirator-style mask when working with polyurethane, no matter what formulation it is.
Polyurethane is inexpensive and easy to apply. A brush is the most common application method, and it is very easy to clean up after application. However, thinned polyurethane can also be applied with an HVLP sprayer, powered by compressed air.
When applying polyurethane, it is important to manage the bubbles present in the surface. Once the bubbles harden, they must be sanded off before another coat of polyurethane can be added. If the finish is still wet, ‘tipping off’, or running the tip of your brush very lightly over the surface, can help pop the bubbles before they dry.
While tints can be added to polyurethane, it is usually applied as a clear coat in at least three or four thin layers. Two thick layers will protect the wood, but the finish may look plastic-like.
When properly applied and cured, it is the most resistant to abrasion of any finish. For this reason, it is often used to coat work surfaces and countertops, chairs and stools, and wood floors.
Polyurethane is heat-resistant, blocks fungus, mildew, rot, and pests, and is also impermeable to water. Contrary to popular belief, this impermeability actually makes it unsuitable for exterior applications.
The moisture trapped inside the wood will expand and contract with changes in temperature and humidity, and a stiff polyurethane film finish isn’t flexible enough to prevent damage.
Varnish vs Polyurethane
Now that you know the basics, it’s time for a deeper dive. Let’s explore the similarities and differences between varnish and polyurethane.
The major similarities between varnish and polyurethane include the way they are applied, their appearance when dry, the level of protection they provide, and the ingredients they contain.
Polyurethane and varnish are applied in the same way. A thinned seal coat provides the base of the finish. Each layer is sanded before another coat is added. Application options include rags, brushes, rollers, and compressed-air powered sprayers.
When applied in thin coats, varnish and polyurethane have a similar appearance. They are usually shiny, rather than matte, although deglossers and abrasion can be used to dull the finish of either product.
Polyurethane that contains no natural resins can often have a plastic-y appearance, but applying multiple thin coats rather than just a few thick layers diminishes this effect.
Both varnish and polyurethane include resins, and dry through the evaporation of solvents. Varnish tends to refer to formulations that include alkyd or phenolic resins. Polyurethane generally contains a blend of more than one urethane resin.
Varnish and polyurethane are exceptionally durable and offer similar amounts of protection when applied properly and allowed to cure. Polyurethane has the edge on resistance to abrasion, while varnish holds up a bit better to UV light and weather. Both are waterproof and repel mildew, rot, pests, and fungus.
Even though polyurethane falls under the category of ‘varnish’, these two finishes have significant differences, including how they accept and hold tint or color. Their varying levels of toxicity, flexibility, and protection from UV light make it possible to distinguish between varnish and polyurethane.
Varnishes formulated with natural oils are less toxic and emit fewer VOCs than oil-based polyurethane. Water polyurethane can be very low in VOCs, or even emit no VOCs at all, but isocyanates are still present in this finish. Although varnish is less toxic than polyurethane, it has a very strong odor.
Polyurethane is a stiff, unyielding finish that repels everything. Varnish is much more flexible than polyurethane, which allows it to be used in non-climate controlled environments where the wood needs to flex and breathe.
Varnish contains a higher amount of solids than polyurethane, which provide superior protection from fading and UV damage. Spar and marine varnish often have UV absorbing materials added.
Polyurethane is generally intended for indoor use and does not offer the same level of UV protection or absorption. There is a spar urethane that has some similar characteristics to polyurethane, and is better used outdoors.
An oil-based stain can be added directly to varnish to change the hue of the finished product. Polyurethane can also be tinted. Rather than adding a stain or paint directly, universal coloring pigments are used to tint polyurethane.
These are the same pigments used to tint paint, and can be purchased at most paint retailers. Either method involves a bit of experimentation, so test your tinted varnish or polyurethane on scrap wood before committing yourself.
Major Differentiating Factor
Varnish can flexibly respond to changes in temperature and moisture, while polyurethane is much stiffer. This makes polyurethane best-suited to interior, climate-controlled environments, while varnish is much better for exterior use.
When to Use Varnish
Use Varnish for Exterior Projects
Wood that will be used outside needs to be flexibly protected. As the temperature and humidity of the air change, so does the amount and volume of moisture inside the wood.
Marine and spar varnishes are more flexible than other finishes, which allows them to protect the wood without cracking or degrading due to the freeze/thaw cycle. These formulations also include protection from UV light.
Varnish’s odor makes it less appealing for use inside. If you’re dead set on varnishing indoors, do it before a vacation to minimize the exposure of your household, or keep the finished piece in the garage for a few weeks before bringing it inside.
When to Use Polyurethane
Use Polyurethane for Interior Projects that Need High-Level Protection from Abrasion, Moisture and Heat
Polyurethane is perfect for dining tables or countertops, as hot dishes and spilled liquids will have no effect on the longevity or appearance of the finish.
Wood floors are also frequently coated in polyurethane due to its superior abrasion resistance. To get the most out of your polyurethaned wood floors, wait at least a few days before moving furniture back into the room. Try to minimize the amount you walk on polyurethane that is dried, but not yet cured.
Which Is Better, Varnish or Polyurethane?
Varnish offers flexible protection that is better than polyurethane for exterior and marine application.
Polyurethane is best for interior projects that require a super-tough layer of protection.