How To Thin Polyurethane for Spraying?

Polyurethane is a finishing coat, used to protect the wood from damage and scratches. It is mostly used for wood finishing; floor finishing and shielded coating. It improves the exterior of the woodwork, giving it a shiny, smooth finish.

For applying polyurethane, rollers and brushes are good tools, but sprayers can work more efficiently. The experts often recommend airless sprayers for applying polyurethane because they provide the highest production, saving your money and time.

Thinning polyurethane like other paints is necessary for spraying. Different types of liquids are used to thin polyurethane to get the right viscosity. You just need to learn how to thin Polyurethane for spraying to get a smooth thin coating.

Why Is It Necessary To Thin Polyurethane for Spraying?

Generally, when polyurethane is prepared and packed in spray cans, it is very thick. So you have to thin it before loading it into the sprayer.

Polyurethane is a plastic liquid and it is difficult to apply it directly to your project. Thinning polyurethane paves the way to getting an even, nice, and smooth finish. So it is necessary to thin it properly for spraying.

Properly thinned polyurethane can seal the wood better and stops the surface from blotches and streaks.

It dries faster because the solvents have a high and quick evaporation rate. But rough regular polyurethane dries slowly because it has a high moisture content which lays longer on the surface before drying.

This article will explore why thinning polyurethane is essential before spraying and how to thin both oil-based and water-based polyurethane.

Main Supplies To Thin Polyurethane for Spraying

Let’s see the main supplies; you need when you are thinning Polyurethane for spraying.

You should have polyurethane, mineral spirit, paint thinner, a glass jar, a clean stick for mixing, clean rags, and a sprayer with you.

What Solvents Are Used To Thin Polyurethane for Spraying?

There are two types of polyurethane and different solvents can be used to thin each.

#1. Water

Water can be used as a solvent to thin water-based polyurethane only. It can be thinned well with water and doesn’t affect the paint’s viscosity.

But water can’t be used to thin oil-based polyurethane because it will make it muddy, sticky, and gummed up.


#2. Denatured Alcohol

Denatured alcohol is used as a solvent to thin polyurethane. It is perfect for thinning water-based polyurethane. But take care when you are thinning polyurethane with denatured alcohol.

#3. Paint Thinner

Paint thinner is a good solvent for thin polyurethane. But make sure, it should be based on the type of polyurethane.

Always use oil-based paint thinner to thin oil-based polyurethane and use water-based thinner to thin water-based polyurethane.

They should not be mixed otherwise you’ll end up blurring the result of polyurethane coating. It will make the clear coat difficult to remain clear even after drying.

#4. Lacquer Thinner

Lacquer thinner is used as a solvent to thin oil-based polyurethane only. It contains oily substances to thin lacquer.

You can’t use it with water-based polyurethane as it will damage the clarity of the finish. Lacquer thinner keeps a stronger odor than other mineral spirits.

#5. Mineral Spirits

Mineral spirits are used as a solvent to thin oil-based polyurethane only. They contain oily resins taken from trees and petroleum distillation. So use mineral spirits only to thin oil-based polyurethane.

#6. Acetone

Acetone is also a solvent but it can’t be used to thin polyurethane. Actually, it is a paint-removing solvent, and it can break up the polymer from the binder.

Acetone can cause the polyurethane to finish too fast and the solvent to evaporate too quickly. If you try to thin polyurethane with acetone, it won’t have an even flow, stick and dry well.

What Is The Proper Ratio To Thin Polyurethane for Spraying?

For thinning oil-based polyurethane, you need to add four parts of polyurethane to one part of the mineral spirit or other solvent i.e. 4:1.

But for thinning water-based polyurethane, you need to add 2 to 3 percent water but it’s not final. It depends on the brands and application of polyurethane. Here you have to add water until you get optimum consistency.

How To Thin Polyurethane for Spraying?

Although both oil-based polyurethane and water-based polyurethane are thinned with different solvents the process is almost the same with a slight change.


So, let’s discuss each step separately to get a clear idea of how to thin polyurethane for spraying.

#1. Choosing A Glass Jar

Although a paint bucket can be used to mix the polyurethane and solvent, you cannot measure the right quantity of the material. So you need a good-sized jar keeping numbers to measure the mixed material well.

You can also see the reaction medium easily, and mix the incorrect ratio of polyurethane and mineral spirit correctly.

#2. Cleaning The Glass Jar

Whatever you use, it must be clean. It is very much essential because the place must be clean, where polyurethane meets mineral spirit and their reaction happen.

The reaction of the solvent and polyurethane should take place in an environment where no interferences happen.

So you must remove dust, stains, paints, and other residues from the glass jar. You can use a clean rag to wash off the stain.

But for hard stains, use warm soapy water. You can also rub it using alcohol and dampen sticky residues.

#3. Opening The Polyurethane Cans

Usually, polyurethane is packed and comes in spray cans. These spray cans have to be cut to take out the polyurethane into the mixing jar.

So for opening the can, you should make a dent in the paint can using a screwdriver. After this take a pair of scissors and cut the paint can. When you have completely opened the can, you can get polyurethane into the clean glass jar.

But, if you have your polyurethane in a paint bucket, just open it and pour it into the glass jar.

#4. Pouring Polyurethane Into The Jar

Sometimes, you buy polyurethane packed in such spray cans, which are designed with thin polyurethane and you don’t need to thin it again.

But if the polyurethane is packed in those spray cans which need thinning, then open the polyurethane can and pour a required amount of polyurethane into the clean glass jar for mixing it.

#5. Measuring And Adding The Paint Thinner

Whatever solvent, you want to use according to the nature and brand of polyurethane, you need to measure it before adding it to the polyurethane.

Whether you are thinning Minwax polyurethane or thinning polyurethane with turpentine, always add paint thinner at the ratio of 1:4.

A measuring cup can also be used for this. You can mix 1/2 cup of paint thinner with one gallon of polyurethane.

You can mix more or less paint thinner according to the application method. If you are applying the polyurethane with a paintbrush, you need not thin the polyurethane as much.

But applying it with a sprayer, the polyurethane should be as thin as possible.

#6. Stirring The Mixture Of Polyurethane

When you have poured both the solvent and polyurethane into the jar, stir the mixture for about 10 minutes with a clean stick.

In this process, try to maintain even pressure on the stick and keep on moving your stick as you write “8” in the mixture.

Stir it smoothly. If you stir too fast, it can cause bubbles in the mixture which can be a great problem. So keep on stirring the mixture to get an even flow.

#7. Testing The Mixture of Polyurethane

When you have prepared well the mixture of polyurethane, test it on cardboard with a sprayer. After the test spray, wait until it dries to show the exact colors.

If there are no imperfections, no bubbles, lumps, or color variations, congratulations, you have thinned the polyurethane rightly.

Now check sprayed polyurethane when it dries, is it good enough? If the paint is thinner, add more polyurethane and stir again. But if the paint is thicker, add more paint thinner and stir again.

#8. Loading The Mixture of Polyurethane Into The Spray Gun

When you have completed the thinning process successfully and got good results in testing it, it’s time for you gentleman to load the spray gun with the Mixture of polyurethane.

Now, start spraying to get a super smooth glossy finish without producing any blotches on the surface. It is very easy to work with thinned polyurethane because it gives excellent results.


Is it necessary to thin polyurethane when spraying?

Yes, of course. The polyurethane comes too thick, and it has to be thinned before spraying. It helps to archive an excellent smooth finish. The thinned polyurethane becomes lighter and sprayed evenly with no harm.

If polyurethane has not been thinned before spraying, it can clog the nozzle of the paint sprayer, and the finish doesn’t come smooth.

Can water-based polyurethane be thinned with mineral spirits?

No, generally, water-based polyurethane doesn’t work well with mineral spirits. So never try to use them to thin water-based polyurethane.

What is the suitable ratio to thin polyurethane for spraying?

For thinning oil-based polyurethane, you need to add four parts of polyurethane to one part of the mineral spirit or other solvent i.e. 4:1.

What is the right pressure to spray polyurethane with a sprayer?

Polyurethane spraying requires 28 PSI as the right pressure to atomize the polyurethane on the surface properly. Typically, it is set between 26 and 29 PSI, particularly when applying clear coats.

If you set pressure lower than 25 PSI, the polyurethane will not be excluded smoothly through the nozzle. As it is thick enough, it requires more pressure to expel polyurethane and apply it to the surface.


Thinning polyurethane for spraying is surely a systematic process. Though it is not difficult, you have to be careful while mixing it with the right solvent.  It is necessary to follow the proper process, such as selecting, cleaning, pouring, stirring, mixing, testing, etc.

If you want to get a good glossy surface, learn how to thin polyurethane for spraying, follow the simple steps explained above, and get the desired result and smooth spraying on the surface.

Polyurethane can be sprayed well through an airless sprayer, whether it is oil-based or water-based.

Just keep your workplace well-ventilated for spraying activities.

Additional Questions

What can be used to dilute polyurethane?

The best way to dilute oil-based varnish or polyurethane is by using mineral spirits. You would usually combine 3 or 4 parts varnish with 1 part mineral spirits. However, the exact ratio isn’t absolutely vital – you can adjust it to fit your needs. As someone who has worked with these materials before, if you like using a rag to apply finishes, I would suggest diluting them with around 50% mineral spirits. This allows you to create a ‘wiping varnish’ that can be applied smoothly and easily.

What is the correct way to thin polyurethane for spray use?

If you’re looking to thin down oil-based polyurethane for spray use, you can use mineral spirits, naphtha, or lacquer thinner. Typically, the mixture should be primarily polyurethane – about 3 parts – with 1 part of solvent. However, if you plan on thinning water-based polyurethane, you can just use water or denatured alcohol. Through my past experiences, I’ve learned not to add more than 2% of additional solvent to the mix.

Can mineral spirits be used to thin polyurethane?

Yes, you can certainly use mineral spirits to thin down your polyurethane. A recommended mixture would be 3 parts polyurethane to 1 part mineral spirits. Be sure to stir your mixture very gently and slowly. Having stirred polyurethane in the past, I know that agitating it too much can introduce air and form undesired bubbles which you’d want to avoid.

Is it possible to use a spray gun for polyurethane?

Absolutely, you can use a spray gun to apply polyurethane. Urethane can be quite tricky to apply smoothly with a brush or roller, so a spray gun really makes a difference. However, it’s important to note that not all types of urethane are suitable for spraying. From my experience, always read the product’s instructions before purchasing new polyurethane. This can save you disappointment and also ensures better results.

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