How to Apply Polyurethane

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Polyurethane is a sealant that protects wood from water, oil, grease, and heat.  When applied, it lends a shiny appearance to the surface of the material. Sealing and protecting with polyurethane is a means of protecting the longevity of wood, as well as beautifying it. 

The protection offered by polyurethane is only as good as the technique used to apply it. If you have something you’d like to protect with polyurethane, this article will lead you through everything you need to know. 

How to Apply Polyurethane to Wood

Polyurethane can be applied to unfinished, stained, or painted wood. Making smart choices about the type, finish, and application method will ensure the success of your polyurethane project. 

  1. Choose a type of polyurethane. There are two types of polyurethane; oil-based, and water-based. Oil-based polyurethanes reveal more of the wood’s character and depth, and tend to look brighter when applied. They tend to be shinier than their water-based counterparts, and can be expected to last longer. For help deciding between oil-based and water-based polyurethane, including environmental concerns, cost, and dry-time, check out the information at the bottom of this article. 
  2. Choose a finish. Generally, three finishes of polyurethane are available to choose from: gloss, semi-gloss, and satin. Some manufacturers also produce a matte option. Like paint on the walls, the shinier and more reflective the finish, the more dirt, scratches, and scuffs will show up. The finish of the polyurethane has no effect on it’s durability. Indoor polyurethanes generally do not include UV protection, so they should never be used for projects that will be exposed to the sun. 
Man preparing to use a can of polyurethane
  1. Prepare your workpiece. Inspect the surface of your project. Use wood filler to hide nail holes and clean up damage. Wipe the wood with a damp cloth to raise the fibers. When it has dried, use 220 grit sandpaper to sand the surface smooth. Vacuum the wood to remove any lingering dust. 
  2. Find an appropriate work space. Polyurethane should not be applied in a dusty area, because dust in the air can get trapped in the polyurethane and stick to the wood. Outdoor application is also difficult, because insects and dirt get blown into the sticky surface while it dries. If you can, move the project inside your home, to a well-ventilated and dust-free area.
  3. Choose your application method. Liquid polyurethane applied with a brush is best for covering large areas, building up a thick layer of protection, and for flat surfaces. Curved or carved areas may require wipe-on polyurethane, which is allowed to penetrate the wood for a predetermined amount of time before being wiped off. Aerosol polyurethane is also available but not recommended unless there is no other option. Applying it evenly is difficult, the finish is thinner than other methods, and you need to protect everything in the area from overspray. 
  4. Apply the polyurethane. If using a brush, use the tips and tricks below to get an even, smooth finish. Wipe-on polyurethane should be rubbed into the wood using a clean, dry, lint-free cloth. After it has penetrated the wood, use a new cloth to wipe off the excess. For aerosol polyurethane, control the rate of the spray carefully. Keep even pressure on the nozzle as you sweep it back and forth over the surface. Don’t overapply; there should not be enough material on the wood to drip or pool. 
  5. Wait for the coat to dry.  Water-based products cure (harden) much more quickly than oil-based polyurethane. About four hours after you apply a water-based polyurethane, your wood will be ready to accept a second coat. If you’re using oil polyurethane, you will need to wait at least 24 hours. 
  6. Clean up imperfections. If there are drips, streaks, or bubbles in the first coat, you need to remove these imperfections before proceeding with the project. 220 grit sandpaper should remove any bubbles or particulates that have been trapped in the polyurethane. For thicker drips, you can cut them off using a razor blade and sand down the surrounding area. 
  7. Apply additional coats. Oil-based polyurethane is composed of 45-50% solid material, which makes it thicker and more viscous than water-based products. As a result, you will likely need only two or three coats. Water-based polyurethane is only 30-35% solids, so four or five coats are usually needed. 

Tips and Tricks for Applying Polyurethane With a Brush

  • Wet your brush. Dipping your brush in water will make it easier to apply water-based polyurethane, and easier to clean the brush afterwards. For oil-based polyurethane, use mineral spirits to wet the brush before using it for application. 
  • Control the amount of material. Dip about ⅓ of the brush into the polyurethane. Tap the bristle base against the rim of the can to remove excess. You want the brush to be fully penetrated, but not dripping. 
  • Apply in long, even strokes. Place the brush at a 45 degree angle at the top of the wood. Stroke the brush in the direction of the grain using one long, even stroke, and transferring the sealant to the wood.  Flip the brush over and apply a second long stroke. Then dip the brush in the polyurethane again. 
  • Keep a wet edge. When applying polyurethane, each brush stroke should slightly overlap the one that came before it for even coverage. If you brush wet polyurethane onto dried or partially dried poly, the finish will pull away from the wood. 

Avoid this by working your way methodically across the surface without taking breaks. Since oil-based polyurethanes take so long to dry, this isn’t usually an issue. Water-based polyurethane dries much more quickly.

  • Avoid bubbles. If air is introduced into the polyurethane, bubbles will form on the surface of your wood. The only way to get rid of these bubbles is to sand them down, and you may need to reapply the polyurethane. 

The two most common ways air gets into polyurethane are shaking the can, wiping the brush against the rim of the can to remove excess product from the brush. To avoid this, never shake a can of polyurethane and use a tapping method to remove excess material. 

What Polyurethane Finish Should I Apply? 

For a heavily trafficked casual living area, matte or satin finishes are generally best. For more formal areas of the home, a semi-gloss finish can appropriately show off your wood floors or furniture. High-gloss will show every speck of dust and the tiniest scuff or scratch, so it is generally reserved for pieces and places that are looked at more than they are interacted with. 

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Should I Wipe-on, Spray or Brush Polyurethane?

There are three ways to apply polyurethane, and you need to pick the correct method depending on your wood surface:
Wipe-on polyurethanes are the easiest to apply, but have less coverage than brush-on applications. It also a very time consuming way to apply poly, but you don’t have any brush marks when you’re done.
Brushing polyurethane is the most common method. Brushing is easy and you can cover a lot of ground quickly. However, you do run the risk of having brush marks show.
Spray-on polyurethanes are trickier and should be used only as a last resort or for hard-to-reach areas. For example, if you’re refinishing ornate furniture or shutters, you might need to spray polyurethane to get all of the cracks and nooks. There are techniques you can use to improve the outcome of brush-on polyurethane. 

Man about to use a spray can to apply polyurethane

Should I Apply Water Based or Oil Based Polyurethane? 

Oil-based polyurethane is not recommended for projects that are made from maple or have been stained white or gray. This is because oil-based polyurethane starts with a vague amber tint that will continue to deepen over time, developing a yellowish hue. While it can be quite attractive, you can avoid yellowing by choosing water-based polyurethane, which will remain clear. 

Choose a water based polyurethane when you want to get the job done quickly. Water-based polyurethane dries in a few hours, while oil-based polyurethane generally needs 24 hours to properly cure, limiting you to one coat per day. Check the manufacturer’s instructions for product-specific guidance.

Choose an oil based polyurethane if you need the finish to last a long time, and hold up to demanding circumstances. Wood pieces going outside should use oil-based products because of the sun and rain. Flooring also needs a oil finish due to how frequently they are walked on.

However, wood floors finished with a water-based product tend to peel, needing refinishing in just five or six years. Oil-based polyurethane does not peel as it ages due to the superior bond with the wood. It can usually last for ten years before a refinish is required.If you’re looking for the longest-lasting results, choose oil-based over water-based polyurethane. 

If you need to refinish a degraded polyurethane, you’ll be glad to know it’s a simple process. Polyurethane only penetrates the top layer of wood. Once you sand this down, you’ve got a blank canvas again. Note that if you want to switch from water-based to oil-based polyurethane, or vice versa, you will need to wait at least six months before doing so.

If you have a short window for refinishing, choose water-based polyurethane. The quicker dry time will allow you to finish the project with a tight turnaround. For longer-lasting results with a correspondingly long dry time between coats, choose oil-based polyurethane. It looks better, costs less, and lasts longer than water-based polyurethane.

Conclusion

Polyurethane is a protective sealant for wood. It is either water-based or oil-based, comes in varying levels of reflective shine, and can be applied with a brush, rag, or spray can, depending on the formulation. 

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Prepare the surface to accept polyurethane by filling any holes with wood filler, then wet the wood and sand it down. When the surface is clean and clear of dust, you are ready to apply the polyurethane. 

After allowing the first coat to dry, remove any imperfections and apply additional coats. The dry time and number of coats needed will vary depending on whether your product is oil or water based.

Ellenkate grew up on job sites run by her family’s construction company. She earned her theater degree from The Hartt School, a prestigious performing arts conservatory in Connecticut. Her design and DIY work from her Chicago loft was featured in the Chicago Reader and on Apartment Therapy.