For those who’ve been practicing woodworking for a while, you’ll probably be plenty familiar with polyurethane and spar urethane. But for those out there looking to start woodworking and searching for the perfect finish for your first table or chair, it can be hard to know which to choose.
So, what is the difference between polyurethane and spar urethane? The main difference between polyurethane and spar urethane is the amount of oil. Spar urethane has more oil, which makes it more flexible and resistant to cracking in outdoor conditions. Polyurethane has less oil, giving it a harder and stiffer finish.
There are a few other differences, such as UV-blocking additives, the level of fumes, and ways to apply. In this article, we’ll explore all these differences and more to help you pick the right finish for your project.
What Are Polyurethane And Spar Urethane?
Before we get into the differences, advantages, and disadvantages of each finish, let’s go over the basics of each finish.
Polyurethane is one of the most commonly used finishes on furniture items. It can also be found in certain types of paint. It’s made from a clear synthetic resin that’s resistant to abrasion and water.
If you’ve seen polyurethane on the shelves at your home improvement store, you’ll know that it comes in both oil-based and water-based options, as well as a variety of gloss finishes.
Spar urethane was originally created as a finish for the spars of a ship, or the wooden poles that support the sails. Because it was designed to withstand harsh weather and elements, it’s more resistant to cracking over time.
One of the best things about spar urethane that makes it the perfect option for outdoor projects is the inclusion of additives that block UV light. This additive helps the spar urethane finish protect your wooden project from fading, yellowing, and wearing down.
Indoor Vs Outdoor Use
Although both finishes can be used in either indoor or outdoor settings, polyurethane is more recommended for indoor use while spar urethane is recommended for outdoor use.
Polyurethane does have the ability to resist water, heat, and cold, but it creates a harder finish. The stiff nature of the finish means that it’s unable to move and warp with the wood as it swells with heat or changes over time.
This doesn’t mean that polyurethane can’t be used for outdoor projects. It’s still a great finish, but it doesn’t stand up as well to the outdoor elements as spar urethane.
If your project involves rubber or metal, however, polyurethane is a great choice as it binds well on these surfaces.
Spar urethane is the best choice when looking for a finish for your outdoor project. The higher amount of oil in the mixture means it’s more flexible and able to adapt to the elements with your furniture.
Don’t think this flexibility makes it a weaker finish, though! Spar urethane is incredibly durable and highly resistant to heat, cold, water, and even UV rays. As we mentioned earlier, spar urethane contains an additive that blocks UV rays and prevents traditional fading and yellowing.
To compare the drying times of polyurethane and spar urethane, it’s important to understand the process each of them goes through to fully dry.
Spar urethane dries through a process called oxidation, which means it dries simply by being exposed to open air. Because it dries just based on air circulation, it tends to dry fairly quickly.
Polyurethane, however, dries much slower. If your project will require multiple coats, you’ll want to keep this in mind to budget your time wisely.
When it comes to polyurethane, one of the advantages is that it’s good for interior use. It creates a shiny, durable finish that helps your newly crafted furniture piece stand the test of time.
It’s also more common, which means it tends to be a little more cost-effective. You can also find it mixed in with paints, which makes it more versatile when it comes to application.
Spar urethane, however, also has many advantages. Some we’ve already reviewed, such as its UV blocking additive and ability to stand up against harsh conditions.
Spar urethane also has a lower amount of toxic fumes than polyurethane, making it a little safer to use. It needs less layers, which combined with its faster drying process makes this finish a real time saver.
Each of the finishes have their own disadvantages, though, and it’s good to know those too so you can choose the best finish for your project.
Polyurethane has a higher amount of toxic chemicals, which can cause headaches, dizziness, nausea, and other health problems.
It’s also incredibly difficult to remove. This could be a good thing if you want your finish to stay on, but if you’re looking to change the stain of a piece of wood furniture already finished with polyurethane, it can be a complicated and messy process.
Spar urethane is more likely to be damaged from other chemicals or alcohol, so be careful what chemicals you use to clean the surface.
Another down side to spar urethane is that it doesn’t mix well with oil-based paints, so isn’t a good added finish to existing paints. Polyurethane doesn’t have this problem.
Finally, spar urethane tends to be a little more expensive. The price difference isn’t so huge that this would be an obstacle, but it’s worth noting for those looking to craft DIY on a tight budget.
Polyurethane can be applied using either a bristle brush or a foam brush. We have another article on the best brushes to buy for applying polyurethane for the best results. You’ll need to use multiple layers to get a solid finish, so keep those brushes handy.
When using polyurethane, be careful with application and keep an eye on your surface. Polyurethane tends to have issues with leaving air bubbles and visible brush strokes in the finish.
If you’ve chosen spar urethane for your project, you’ll want to stick with the bristle brush and avoid the foam brush. Spar urethane has a thicker consistency, which doesn’t apply as easily and needs the looser applicator that bristles provide.
Both of these finishes can be applied by wiping on with a clean rag rather than brushing on, but this process involves diluting the finish with other chemicals, which means you’ll need to apply more layers than you would by brushing on.
They can also both be available in spray can form. Polyurethane sprays on fairly well, but spar urethane is more difficult to spray on due to its thick consistency.
When using either spar urethane or polyurethane, it’s vital to keep in mind that they’re both flammable products. Keep a fire extinguisher nearby and don’t apply them near heat or open flame. This includes candles!
Always wear proper safety gear, such as a ventilated mask and gloves, when applying either finish. Regardless of the lower level of toxic chemicals in spar urethane, they both still contain toxic chemicals.
Because of the toxic chemicals, we recommend working in a well-ventilated area. This can help alleviate any potential health symptoms you may experience, such as headache or dizziness.
If you start to experience these symptoms or any other health issues, take a break in an area with cleaner air to help circulate fresh air into your body.
Which To Choose?
When it comes to choosing a finish for your next project, ask yourself these questions:
- Will your project primarily stay indoors or outdoors? For indoors we recommend polyurethane, and for outdoors we recommend spar urethane.
- Do you need the flexibility of spraying on your finish? If so, polyurethane is the better option.
- Are you someone who’s sensitive to chemical smells? Try spar urethane, as it has less toxic fumes.
- Do you have an upcoming deadline and need to finish the project soon? Spar urethane dries much faster and needs less coats, helping you to finish your project faster.
Can I Use Polycrylic Instead Of Polyurethane?
Although their names seem similar, they’re certainly not the same.
Polycrylic is an excellent finish option, especially in spray-can form, but the consistency is very runny. This means you’ll have to keep an eye out for any drips after application and between coats.
It also dries extremely quickly, making it a little difficult to use on large surfaces but convenient to use for items that need finished quickly.
However, polyurethane is said to have a yellow tint due to the oil in the mixture, which can alter the final look of your project. Polycrylic has a true clear finish that won’t change your desired color.
Polycrylic is significantly less toxic than polyurethane. It doesn’t have strong toxic fumes like polyurethane and it’s not nearly as flammable, making it much safer to work with.
Where budget is concerned, polycrylic can be a cheaper option than polyurethane.
Finally, polycrylic is much easier to clean up. To remove it from your hands, the application brushes, and any other surface or object, you simply need to wash with soap and water.
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