How to Spray Polyurethane? (5 Steps)

Spraying paint has always been better and faster than using a paintbrush. So, how to spray polyurethane?

To spray polyurethane, you need a paint sprayer or a spray gun. You can fill the paint sprayer with polyurethane and spray it. The polyurethane will come out of the sprayer nozzle. You can also use spray-on polyurethane. 

The spray-on polyurethane comes in a spray bottle, so you can spray it without prep work. However, you shouldn’t over-thin polyurethane as it can lose its gloss finish. 

So what type of polyurethane can you spray, and what type shouldn’t you spray? Also, at what pressure should you spray polyurethane? The answers to these questions and more will be revealed in this article. So let’s dive in.

Can You Spray Polyurethane?

You can spray polyurethane with a spray gun, an airless paint sprayer, or a compressed paint sprayer. All of these are painting equipment that are designed with a paint cup. You can pour the polyurethane into the paint cup and pull the trigger to spray the polyurethane through the nozzle of the spray gun.

However, there are a few things to know before you spray polyurethane:

  1. You must thin polyurethane before spraying – Polyurethane is thick, and in its thick state, it would be difficult to get it flowing through the nozzle of the spray gun. So, you must thin polyurethane before spraying to make it light enough so it can flow through the paint sprayer nozzle.
  2. Spray-on polyurethane is better This is because you don’t need to thin or stir spray-on polyurethane. In most cases, you just need to shake the can for a few seconds.
  3. You can’t spray all types of polyurethane – Technically, if you thin it enough, you can spray all polyurethane types. But, oil-based polyurethane is thick, and if you heavily thin it, the oil-based polyurethane will lose its natural gloss and hard finish. So, over-thinned oil-based polyurethane won’t protect your wood.

Oil-based vs Water-based Polyurethane (For Spraying)

Should You Spray Oil-Based Polyurethane or Water-Based Polyurethane?

You can spray water-based and oil-based polyurethane. But, water-based polyurethane is easier to spray than oil-based polyurethane.

Here is why:

Paint Flow

The paint flow is the most important factor when picking a polyurethane to spray. This is because the flow determines if the polyurethane is easy to spray or not. 

Water-based polyurethane has a thin flow because it uses water as its solvent, so the paint particles are light or thin. On the other hand, oil-based polyurethane has a thick flow because it uses oil as its solvent. Since oil is thicker than water, oil-based polyurethane will have a thicker flow than water-based polyurethane. 

As such, water-based polyurethane is easier to spray. 

Paint Thinner

Regardless of the type of polyurethane, you must always thin the polyurethane before spraying. However, it’s easier to thin water-based polyurethane than oil-based polyurethane. 

To thin water-based polyurethane, you must use water. But, to thin oil-based polyurethane, you must use mineral or petroleum-based solvents, such as mineral spirits. 

Also, since oil-based polyurethane is thick, you must use a lot of paint thinner to thin it. Since you will use too much paint thinner, you can easily over-thin oil-based polyurethane. Over-thinning oil-based polyurethane can take away its natural and glossy finish.

Related Read: Oil-Based vs Water-Based Polyurethane?

How To Spray Polyurethane?

To spray polyurethane, you’ll need a few tools and supplies:

  • Clean rags
  • Sanding sealer (for if the surface is bad or rough)
  • Sandpaper
  • Paint sprayer or spray gun
  • Spray shield
  • A pair of gloves
  • A face mask
  • Drop sheets
  • Painter’s tape
  • Mineral spirits or paint thinner

1. Prep Your Workspace

Prep Your Workspace

First, prep the workspace:

Put a large drop sheet on the floor to prevent polyurethane from staining the floor. You can hold the drop sheet down on the floor using painter’s tape so it doesn’t move out of the place while spraying. Also, use painter’s tape to cover areas you don’t want polyurethane. You must also put on a pair of gloves and a mask.

2. Sand The Surface

Sand The Surface

If the surface is rough or uneven, you must apply sanding sealer over the surface. The sanding sealer will condition the surface so the polyurethane is applied smoothly and evenly. 

After you apply the sanding sealer, you must sand the surface with a fine-grit sandpaper. Sanding will smoothen the surface and help the polyurethane to stick better. 

3. Thin The Polyurethane

Thin The Polyurethane

Next, thin the polyurethane. Thinned polyurethane can flow easier through the nozzle of the spray gun. The polyurethane will clog the spray gun nozzle if you spray un-thinned (thick) polyurethane.

To thin water-based polyurethane, use water. To thin oil-based polyurethane, use mineral spirits or lacquer thinner. You must use a ratio of 4:1 (4 parts polyurethane to 1 part paint thinner) to thin polyurethane. With this thinning ratio, the polyurethane will have a thin flow but still maintain its hard and moisture-resistant finish.

You should never switch the paint thinners. This means you shouldn’t thin oil-based polyurethane with water or water-based polyurethane with mineral spirits. If you do, you will ruin the polyurethane’s color, consistency, and flow.

4. Prep The Paint Sprayer

Prep The Paint Sprayer

To prep the paint sprayer, you must clean and wipe the sprayer. Then, you must use the right tip to attach to the paint sprayer. For polyurethane, use a sprayer tip between 1.0 and 1.5mm. 

Also, you must prime the sprayer; pour polyurethane into the paint sprayer, and switch the multiport valve to the primer. Once the polyurethane starts flowing through the nozzle, the sprayer is fully prepped and you can use it. 

For spray-on polyurethane, prepping isn’t required. You just need to shake the spray-on polyurethane for a few seconds and use it. While shaking the spray-on polyurethane, the polyurethane inside will mix and stir. 

5. Spray the Polyurethane

Spray the Polyurethane

After the paint sprayer is prepped, test the sprayer to see if the finish is smooth and if it dries well. If the spray polyurethane has a consistent flow and dries well, you can use it. 

To spray polyurethane, use a pressure of 28 PSI. Generally, clear coats, such as polyurethane, must be applied with the spray gun pressure set between 26-29 PSI. At this pressure, polyurethane can be well atomized (or sprayed) to the surface. 

If you spray polyurethane at a lower pressure than 25 PSI, the polyurethane won’t expel smoothly from the nozzle. Since polyurethane is thick, you must use more pressure to expel it smoothly on the surface. 

While spraying polyurethane, cover the area with overlapping sprays. Keep the paint sprayer nozzle 10-inches away from the surface to prevent splashes. Also, apply thin coats of polyurethane spray.

You need 2-3 coats of spray polyurethane for proper coverage. You must allow the spray polyurethane to dry for 3 hours before recoating. You also must sand between coats of polyurethane with 320-grit sandpaper. The final coat doesn’t need sanding. 

After you spray polyurethane, you must let it dry for 3 days before you use or clean it. 

Is Spray Polyurethane Good?

Polyurethane spray paint is a great finish and works well on different surfaces, including wood. Polyurethane offers moisture resistance and scratch resistance features. In addition, exterior polyurethane offers weather and UV resistance to the surfaces.

Polyurethane is one of the strongest and best finishes in the painting world. The word “urethane” is a plastic-like material that dries hard. When the polyurethane dries, the liquid layer becomes a very hard and moisture-resistant finish.

The hard layer prevents scratch marks, scars, claw marks, and dents from reaching the surface underneath. The hard layer also prevents moisture and solvents from penetrating.

Spray Polyurethane vs Brush Polyurethane

Spray polyurethane is better than brush-on polyurethane. That’s because spray polyurethane is easier to apply and gives a better finish than brush-on polyurethane.

Since spray polyurethane is easy to apply, it can be used by beginners. Brush-on polyurethane will reveal brush marks and strokes if applied by a beginner. 

Here’s why spray-on polyurethane is better than brush-on polyurethane:

  1. Spraying polyurethane offers better coverage over the surface
  2. You don’t need to prep spray polyurethane, while brush-on polyurethane must be thinned, stirred, and left to settle. 
  3. You can cover a surface faster with spray polyurethane than brush-on polyurethane.
  4. Spray polyurethane covers the surface evenly
  5. Brush-on polyurethane reveals brush marks, but spray polyurethane goes on smoothly
  6. Spray polyurethane is easier to apply
  7. Spray polyurethane offers a better finish quality. 

However, brush-on polyurethane is not a bad choice. In the hands of experienced painters, you’ll get the same result as spraying the polyurethane.

Final Words

You can use a spray gun to spray polyurethane coating. But, you must pick the right type of polyurethane and the right sprayer nozzle. Water-based polyurethane is better than oil-based polyurethane for spraying. 

As an alternative to a paint sprayer, you can use spray-on polyurethane. 

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