How To Thin Polyurethane

Polyurethane that is too thick won’t spread well. It also takes a long time to cure and leaves behind unsightly bubbles and wrinkles. I’ll explain when you should thin polyurethane, and when it’s better to leave it as is. You’ll also learn what can be used to thin polyurethane, and get step by step instructions so you can do it yourself at home.

How to Thin Polyurethane

In order to spray polyurethane with a spray gun, you will need to thin it. If you are brushing it on, thinning can help you avoid common problems like bubbling, wrinkling, and visible brushstrokes. 

1. Consider your ventilation. Polyurethane and thinner are both highly toxic chemicals. If inhaled, they can be dangerous to your health. Make sure you have a constant supply of fresh air in your workspace by opening doors and windows and turning on fans. Note that if you have an air conditioner, you should make sure it is set to bring in new air from outside rather than recycling the air in the room. 
2. Protect your body. A well-fitting, respirator-style mask that covers your face and nose is essential when thinning polyurethane. Nitrile gloves and long sleeves protect your skin. Use goggles to keep these harsh chemicals out of your eyes in the event of a splash or dropped container.
3. Decide on a thinner. There are two kinds of polyurethane; water-based and oil-based. Both can be safely and effectively diluted with various solvents and chemical thinners. Water-based polyurethanes can sometimes be diluted with water. See the section below for more information about the available options.
4. Mix. For the first coat, 50% dilution is recommended. Pour the thinner of your choice down the side and into the bottom of a clean container. Tilting the container as you pour will help minimize the amount of air that gets into the mixture, preventing bubbles from forming. Using the same pouring technique, add the polyurethane to the container. Stir gently with a wooden stir stick.  
5. Apply the seal coat. The first coat seals the wood and forms the base of your finish, so it is particularly important that it be bubble and drip free. If not, you risk carrying those defects forward in every layer of polyurethane that follows. To avoid bubbles, use slow brushstrokes and a natural bristle brush in place of a synthetic exploded bristle brush.

6. Remove bubbles. Keep an eye on the seal coat while it’s drying. Any bubbles should pop by themselves. If they don’t, pop them gently with your brush using the ‘tip off’ method. If, 15 minutes after application, the polyurethane is still heavily bubbled you can wipe off the bumpy layer using a rag coated in turpentine, mineral spirits, or naphtha. This method won’t work after about 30 minutes of dry time. In this case, you’ll have to sand the bubbles off instead.
7. Adjust your environment. If the seal coat bubbles excessively, you may be working in weather that is too hot. Try using fans to cool the area, or wait for a cooler day. If the seal coat took much longer to dry than expected, it’s probably too cold and you need to move to a warmer, drier space.   
8. Mix. For the middle layers of polyurethane, opinions vary about how much the finish should be diluted. Some experts argue that you don’t need to thin the polyurethane at all unless you want to speed dry time. Others point out that thinned polyurethane is less susceptible to brushstrokes and collects less dust while drying. At the end of the day, it’s a matter of personal preference. With a little practice, you’ll figure out what works best for you. 
9. Build the finish. Apply several coats of polyurethane until the finish is the thickness you desire. Wait for each coat to dry to the touch before adding another. Sand between each coat to remove dust that settles on the sticky surface as the polyurethane dries. 
10. Mix. For the final coat, quick dry time is very valuable. The longer the surface stays wet, the more dust it will collect while drying. High-gloss polyurethane will show every speck of dust. Satin and matte polyurethanes are more forgiving. Try diluting the polyurethane by 5 or 10% for a thicker coat that still dries more quickly than unthinned polyurethane. Depending on the weather, you may want to use more or less solvent. 

Should I Thin Polyurethane on Hot/Cold Days? 

The curing process is affected by temperature. Thinning polyurethane is one way to compensate for less than ideal environmental conditions. 

On hot days, thin sparingly, and try to keep the piece out of direct sunlight. Turpentine and paint thinner evaporate more slowly than naphtha or mineral spirits, so they can also be used to slow dry time. When the air temperature around the polyurethane is too high, the surface of the poly can ‘skin over’. As the material underneath the skin shrinks, you are left with unsightly wrinkles. 

On the opposite end of the spectrum is polyurethane that dries too slowly or doesn’t cure. Finishes applied below 55 degrees Fahrenheit often develop an ‘orange peel’ appearance as the finish does not level properly, no matter how much solvent you add. Rather than thinning your polyurethane on a cold day, try to create a warm and dry space to apply it, or at least bring it inside to dry. 

What Can I Use to Thin Polyurethane? 

A range of options are available to thin polyurethane. Everything mentioned here can be found at your local hardware store or home improvement center. 

Solvents

The following liquids can all be used to dissolve some of the bonds between molecules of polyurethane, creating a less viscous liquid that is easier to apply and dries more quickly than unthinned polyurethane. These solvents are generally referred to as ‘paint thinners’. 

Turpentine

This solvent has the lowest evaporation rate. It will take considerably longer than mineral spirits or naptha. It also has a strong aroma that lingers. Overexposure to turpentine can cause dizziness and skin irritation, among other symptoms. 

Mineral Spirits

Useful for cleaning brushes and rollers, mineral spirits are a workshop staple. They are the most popular choice for thinning polyurethane. The evaporation rate is neither too long nor too short.

While there is a strong odor, it clears out much more quickly than the stench of paint thinner and is less offensive to the nose than turpentine. Because they have been heavily refined, mineral spirits are also lower in VOCs than less-refined paint thinners.

Naptha

If you want to speed up the dry time of your polyurethane, naptha is the best choice. It’s more powerful than mineral spirits, so you may want to adjust the ratio to reflect this. Naptha should not be used in hot environments, as it will cause the polyurethane to dry too quickly. 

Varnish Thinner

Though the names are similar, varnish thinners are very different from paint thinners. For one thing, they are much more toxic. Take great care to avoid getting varnish thinner on your skin or in your eyes. It is particularly important to wear a well-fitting respirator-style mask when using varnish thinner due to the high levels of VOCs present. 

Another difference between solvents and thinners is the mechanism by which they work. Instead of dissolving the bonds between molecules, a true varnish thinner forces the molecules apart. This has no effect on the evaporation of the solvent, so using varnish thinner to dilute polyurethane will not affect the dry time.  

Water

If you have a water-based polyurethane, check the label. Similar to thinning wood stain, you may be able to thin the polyurethane simply by adding water. Water-based poly is already thinner than oil-based polyurethane, so you may not need to add much. Start with a 5% dilution and move to 10% if that isn’t enough. 

Does Thinned Polyurethane Dry More Quickly? 

Thinned polyurethane does dry quicker, but only by small amount.  

Polyurethane finishes work in two phases. In the first phase, the solvent evaporates and the material shrinks due to the loss of moisture. At this point, the polyurethane will feel dry to the touch and you can add another coat. 

Adding more solvent to your polyurethane will speed up the time it takes for this first phase to complete. Each individual layer will dry more quickly, but you may need to apply a greater number of coats, which can add to the total project timeline. The solvent you use also makes a difference – naptha evaporates more quickly than other solvents. 

In the second phase, the polyurethane hardens or ‘cures’ due to a phenomenon called crosslinking. Oxygen molecules bond to the molecules in the polyurethane and create strong bonds that repel liquids and protect wood.

For oil-based polyurethane, this process takes about 30 days to complete. Water-based finishes cure more quickly, taking just two to five days. Thinners and solvents won’t do anything to speed up this second phase. 

Conclusion

Thinning polyurethane is necessary when applying it with a spray gun, and optional when it is applied with a brush. A 50/50 mixture of polyurethane and thinner is used for the first coat. Later coats can contain between 5% and 25% thinner.

The amount and type of thinner used depends on your goals for the project, the environment you’re working in, and the kind of polyurethane you choose.