How to Spray Polyurethane? Photo Guide

You can use a standard paint sprayer equipment to spray polyurethane. However, to achieve good results, prepare the workpiece before spraying, thin the polyurethane for better coverage, and sand between coats to remove imperfections. Generally, water-based polyurethane is best as it is easier to clean.

A step-by-step guide on spraying polyurethane to get a good finish and improve the productivity of your project.

Spraying Polyurethane: Is it Better than Using a Brush?

Comparing the finish of sprayed PU against brushed PU
Comparing the finish of sprayed PU against brushed PU

Spraying polyurethane is a better option than using a brush for several reasons. 

  • It results in a smoother, more uniform finish with fewer brush marks and bubbles. 
  • It’s a faster method, as it covers large surfaces quickly and efficiently. 
  • It allows for better control over the thickness of the finish, which can be adjusted by adjusting the spray nozzle or thinning the polyurethane.
  • Spraying polyurethane reduces the risk of overbrushing, which can cause the finish to dry too quickly, resulting in a poor surface appearance. 
  • Spraying polyurethane can be more economical than using a brush, as it uses less material and requires less time for cleaning up. 

However, spraying involves a large setup time and is suitable only when working on large projects where quick operation time will compensate for the setup time.

How to Spray Polyurethane?

Surface finish of sprayed PU on the workpiece
Surface finish of sprayed PU on the workpiece
1Prepare your Workpiece
2Setup a Dust-free Environment
3Setup the Sprayer
4Thin or Mix the Polyurethane
5Spray the Polyurethane
6Sand Between Coats
7Apply More Coats
Steps involved in spraying polyurethane

Step 1: Prepare your Workpiece

Sanding the wooden workpiece to get a smooth surface finish
Sanding the wooden workpiece to get a smooth surface finish

Before spraying polyurethane, make sure that your workpiece is properly prepared. 

Begin by sanding your workpiece with 220-grit sandpaper to ensure smooth and even surface.

Sand along the grain to prevent scratches, and make sure to remove any dust or debris. 

After sanding, use a tack cloth to wipe the surface and remove any remaining dust particles.

Next, check the surface of the workpiece for any defects, like cracks or holes, and fill them using a suitable filler. 

Once the filler has dried, sand the surface again with 220-grit sandpaper and wipe it clean with a tack cloth.

If your workpiece has any imperfections that you want to hide, cover it with a stain or dye before spraying the polyurethane. 

You can also use a sealer to prepare the surface for the polyurethane.

If you’re unsure, test the surface of the workpiece with water. If the water beads up, you don’t need a sealer, but if it soaks into the surface, use a sealer to prevent the polyurethane from soaking into the wood.

Finally, make sure that your workpiece is completely dry before applying the polyurethane. 

Moisture can interfere with the adhesion of the polyurethane and lead to an uneven finish or cracking.

Step 2: Setup a Dust-free Environment

Before spraying polyurethane, you must ensure that your work environment is free of dust and debris.

Even the smallest dust can ruin your workpiece’s final finish.

Start by cleaning the room and sweeping the floors. You can also use a vacuum cleaner or a dust collector to remove debris from the air.

If possible, choose a room with minimal foot traffic and keep the doors and windows closed during spraying. 

Also, use plastic sheets or tarps to cover any nearby furniture or objects to prevent dust and debris from settling on them.

Step 3: Setup the Sprayer

Setting up the sprayer for the process
Setting up the sprayer for the process

Once you have selected the right sprayer, the next step is to set it up for spraying polyurethane.

Start by attaching the hose to the sprayer and connecting it to the air compressor. 

Then, make sure that the compressor is set to the correct pressure.

Next, you need to attach the spray tip to the gun.

Most spray guns come with a selection of spray tips of different sizes and shapes, so you can choose one that is appropriate for your job. 

For spraying polyurethane, you’ll typically want to use a fan tip, which sprays a wide, fan-shaped pattern of polyurethane, providing a smooth finish.

Once you have attached the spray tip, you must adjust the air and fluid controls on the spray gun or paint sprayer.

The air control regulates the volume of air that flows through the gun, while the fluid control regulates the flow of polyurethane.

You’ll need to adjust both controls until you get the right combination of air and polyurethane flow to achieve the desired finish without forming any air bubbles.

Test your sprayer on a piece of scrap wood before you start spraying your actual workpiece.

If you can achieve a perfect finish with your current setup, you can move to the actual workpiece.

Step 4: Thin or Mix the Polyurethane

Right proportion for thinning Polyurethane
Right proportion for thinning polyurethane

Although it is possible to spray polyurethane without thinning it, thinning it will result in a smoother and more consistent finish.

You can use water as thinning solvent for water-based polyurethane, while oil-based polyurethane can be thinned with paint thinner, mineral spirits, or lacquer thinner. 

Generally, polyurethane and thinning solvents are mixed in a ratio of 3:1, but you can adjust them to meet your desired result.

Mix a small amount of color into the polyurethane before spraying when spraying over dark colors. This will help prevent any uneven textures, fogginess, or discoloration. 

After thinning and preparing your poly mixture, filter it to remove any lumps or debris which can clog the spraying nozzle.

Pouring thinned polyurethane in the sprayer after straining
Pouring thinned polyurethane in the sprayer after straining

You can also use a water-based polyurethane spray to avoid all the hassle. It comes ready to be sprayed. You can shake and spray it.

Step 5: Spray the Polyurethane

Spraying Polyurethane on the workpiece
Spraying polyurethane on the workpiece

To spray polyurethane evenly, hold the sprayer 6 to 10 inches away from the surface and maintain a consistent distance.

Spray continuously back-and-forth, perpendicular to the workpiece, and overlap your passes.

Apply thin coats rather than one heavy coat, and allow each coat to dry before applying the next.

Step 6: Sand Between Coats

Sanding polyurethane surface
Sanding polyurethane surface

Sanding between coats is an essential step to achieve a smooth finish.

Gently sand the surface with 400-grit sandpaper after the first coat is dry, being careful not to sand too hard.

Remove dust particles with a tack cloth before applying the next coat.

Step 7: Apply More Coats

Spraying more coats of polyurethane
Spraying more coats of polyurethane

Apply at least two coats of polyurethane for a durable, even finish.

The first coat may leave some areas uncovered or poorly covered.

The second coat fills in gaps and protects from scratches, scuffs, and wear and tear.

After spraying, clean the sprayer properly to prevent clogging.

Selecting the Right Polyurethane for Spraying

Water-based and oil-based polyurethane
Water-based and oil-based polyurethane

When selecting the right polyurethane for spraying, there are a few factors to consider. 

1. Type of Polyurethane

There are two main types of polyurethane that you can use for spraying: oil-based and water-based

Oil-based polyurethane is durable but slow-drying, while water-based polyurethane is quick-drying and easier to clean up.

2. Type of Sheen

Polyurethane is available in different sheens, including gloss, semi-gloss, and satin. 

Gloss polyurethane is most durable but shows scratches, while satin polyurethane is forgiving of imperfections.

On the other hand, Semi-gloss offers a balance between durability and sheen.

3. Brand and Quality

Finally, choose a good quality polyurethane from a reputable brand to ensure the best results. 

Cheaper, low-quality polyurethane may not provide good coverage, may not adhere well to the wood, and may not provide the desired level of protection.

Which Type of Equipment is Best Suited for Spraying Polyurethane?

Sprayer equipment for polyurethane
Sprayer equipment for polyurethane

To choose the best polyurethane sprayer, consider your needs and preferences.

Airless sprayers are fast and efficient for large projects but can be difficult to control.

HVLP sprayers produce a softer spray pattern and reduced overspray but are slower and may require an air compressor.

Conventional sprayers are precise and produce a fine finish but are slower and may produce overspray.

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Frequently Asked Questions (FAQ)

Is it necessary to thin polyurethane before spraying it?

No, it is not necessary to thin polyurethane before spraying. However, thinning the PU helps achieve a smoother and more consistent finish, especially when using entry-level spraying equipment. Generally, water-based polyurethane can be thinned with water, while oil-based polyurethane can be thinned with mineral spirits or lacquer thinner.

Can any spray gun be used for spraying polyurethane?

No, not all spray guns are suitable for spraying polyurethane. It’s important to use a spray gun specifically designed for this purpose, such as an HVLP (High Volume Low Pressure) spray gun. These guns are designed to atomize the polyurethane effectively and deliver a uniform spray pattern.

How do I prevent dust from settling on my workpiece while spraying polyurethane?

To prevent dust and debris from settling on your workpiece while spraying polyurethane, it’s important to set up a dust-free environment. Start by cleaning the room thoroughly, sweeping the floors, and using a vacuum cleaner or a dust collector to remove any debris from the air. You can also use plastic sheets or tarps to cover nearby furniture or objects.