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Difference Between Polyurethane and Varnish

Varnish and polyurethane are the most popular wood finishes used to add a shiny gloss and protection on different types of woods.

Varnish is the old type of finish and, oftentimes, most types of finishes are mistakenly referred to as vanish even those that are entirely polyurethane.

This comparison highlights the difference between these two wood finishes.


What is polyurethane?

Polyurethane is an oil- or water-based wood finish that is mainly a plastic resin that hardens when applied on the surface. There also water-based oil-modified polyurethane that stands in between oil-based and water-based. Compared with the water-based one, the water-based oil-modified polyurethane offers a robust protection, and durable surface. Oil-based polyurethane is more durable and offers good protection on woods even though it is more toxic.

Polyurethane might appear milky in its container but it is clear when applied. It is thick and as thus requires a few layers unlike varnish which requires several layers because it is thin. Polyurethane, particularly the oil-based one, is more scratch and abrasion resistant on hardwood. It is, however, applied predominantly on indoor projects due to its weak protection against UV damage. Modern oil-based polyurethane products have improved UV damage but still lacking behind vanish in that UV protection.

The choice of which polyurethane to use may depend on personal preference. Water-based polyurethane is best suited for indoor wood products such as desks, book shelves, and wood floors that need a final finish. It is less toxic and cleans easily with just water and soap, so it could be a better option for anyone allergic to the toxicity of oil-based polyurethane. However, the water-based polyurethane is more prone to cracks if exposed to heat or the sun. As such, it is not advisable to apply on any outdoor furniture exposed to the sun.

Oil-based polyurethane is heat-tolerant so it may survive exposure to sunlight but not for a prolonged time. It is, however, toxic and dries slowly yet expensive.

Polyurethane is best applied with a brush, spray, and foam-roller or by wiping it using hands. It matters not whether it is water-based or oil-based. If applying polyurethane on an existing oil-coat, sand the existing coat so that the new coats can stick firmly onto it. Because of its extent of toxicity, you need sufficient ventilation when applying oil-based polyurethane. Avoid inhaling it or any contact with the skin. If you have existing breathing problems, inhaling oil-based polyurethane could aggravate the conditions. Always use a respiratory musk when applying it. Water-based polyurethane is thinner than oil-based.


What is varnish?

Varnish is often a mixture of resin, solvent and oil. It is a natural wood finish best applied on outdoor wood furniture because of its resistance to UV and water damage. There is a high-content of solids in varnish which makes it a weather-resistant coating. When applied, it leaves a glossy and thin texture usually with a slight yellow color more like oil-based polyurethane.

Varnish is best suited for outdoor decks, garden seats and any other wood furniture decorating the backyard. It is less toxic compared to oil-based polyurethane. Also, it is more flexible, meaning, unlike polyurethane, it is not susceptible to breaking if the furniture is more from one place to the other. It is applied mostly on softwoods.

When applying varnish, use brush to apply numerous layers because it is thin. The downside of varnish is that each layer works on its own, meaning they do not stick firmly unless the previous layer is sanded or scrapped with steel wool to leave a rough surface. If applied incorrectly, varnish can be susceptible to damages such as peeling or cracking. Avoid shaking the tin to prevent bubbles when applying.


Key differences between polyurethane and varnish

Composition of polyurethane and varnish

Varnish is composed of resins, solvents and oils whereas polyurethane is available as water-based, water-based oil-modified and oil-based polyurethane.

Durability of polyurethane vs. varnish

Polyurethane is more durable and harder. It cures into a hard layer of plastic to prevent against scratches and abrasion. The oil-based polyurethane is more durable compared with the water-based. But, the hard surface of polyurethane is restricted to indoor furniture and fixed positions of the furniture. Moving the furniture around and exposing it to the sun compromises its durability because of its hard surface as it could crack.

Varnish is less durable but it is more versatile and flexible. It is more durable on outdoor furniture than polyurethane because of its UV damage protection and water-resistance.

Toxicity in polyurethane and varnish

Varnish is less toxic when compared to polyurethane. Oil-based polyurethane is more toxic than both water-based polyurethane and varnish.

Polyurethane Vs.  Varnish: Comparison Chart


Summary of Polyurethane Vs.  Varnish

  • Polyurethane is available as oil-based or water-based, or even water-based oil-modified
  • Varnish is an old wood finish comprised of oils, resins and solvents
  • Polyurethane is generally more durable but prone to damage if exposed to UV damage or water
  • Varnish is less durable but lasts long when exposed to UV rays and water
  • Varnish is applied by brush and with several layers because it is thinner
  • Polyurethane is applied by wiping it, brush or spray and with few layers because it is thicker
  • Oil-based polyurethane is more toxic than water-based and varnish
  • Polyurethane requires good ventilation when applied
  • Polyurethane is more expensive than varnish
  • Varnish is more applied on softwoods while polyurethane is more applied on hardwoods.

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References :

[0]Image credit: https://www.flickr.com/photos/49364825@N02/4526774736

[1]Image credit: https://upload.wikimedia.org/wikipedia/commons/thumb/0/09/Varnish.jpg/622px-Varnish.jpg

[2]Bob Flexner (2010). Understanding Wood Finishing: How to Select and Apply the Right Finish. Fox Chapel Publishing Company, Incorporated

[3]Mick Allen, Paul Forrester (21 Mar 2006). The Complete Guide to Wood Finishes: How to Apply and Restore Lacquers, Polishes, Stains and Varnishes. Simon and Schuster Publishers

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