Can You Apply Polyurethane on Plastic? (How-To)

Polyurethane is a moisture-resistant clear coat that is used as a sealant on different surfaces to improve durability.

But can polyurethane be used on plastic? Here is the answer to that.

Polyurethane can be used on plastic as long as the plastic surface is well-prepped before the polyurethane is applied.

However, polyurethane is not the best finish to use on bare plastic. This is because plastic surfaces are not porous and the polyurethane has very little to adhere to. This will cause the polyurethane to peel off.

Polyurethane is more suited to painted or primed plastic rather than bare plastic.

But that’s just a snippet. This post reveals more about using polyurethane on plastic. So let’s dive in.

Does Polyurethane Stick To Plastic?

Polyurethane doesn’t stick to bare plastic because plastic surfaces are not porous. The slick and impermeable nature of plastic makes it impossible for polyurethane to penetrate and stick to the surface properly.

For polyurethane to be applied on plastic, the plastic surface has to be sanded repeatedly to create holes and scratch marks that the polyurethane can bite into.

But even after being sanded, the polyurethane will still not sit on the plastic properly. Polyurethane applied on bare plastic will not last up to a year.

To understand why polyurethane will not stick to bare plastic properly, you need to know how polyurethane sticks to surfaces. When polyurethane is applied on a surface, the clear coat sinks into tiny slits, cracks, and fibers on the surface.

On wooden surfaces for instance, the polyurethane will sink into the wood fibers and grains. This allows the polyurethane to adhere tightly to the wood because it can be absorbed into the wooden material. Since the polyurethane has soaked and dried in the wood, the clear coat will stick.

Plastic surfaces on the other hand don’t have any grain, fiber, or slit. Plastic surfaces have a very similar texture to polyurethane. When polyurethane dries, the finish is slick and sealed with no slits, grain, or fiber. This is why polyurethane is widely referred to as a plastic finish.

So when polyurethane is applied on a bare plastic surface, the polyurethane will have nothing to bite into. The lack of grain or fiber makes it very difficult for polyurethane to stick on the plastic.

However, polyurethane will stick to painted plastic as long as the existing paint on the plastic is lightly sanded.

Related Read: Does Chalk Paint Stick To Plastic?

How Long Does Polyurethane Take To Dry On Plastic?

It takes between 1 and 4 hours for polyurethane to dry on plastic. On average, polyurethane will dry 2 hours after its application on plastic.

Polyurethane will however take several days to cure on plastic. The polyurethane cures when the solvent is fully evaporated from the paint particles and the paint pigments harden to form an ultra-thick finish.

On average, polyurethane will fully cure 3 weeks after its application on plastic. The time it takes for polyurethane to dry on plastic depends on the base of the polyurethane and the condition of the plastic.

Usually, water-based polyurethane dries faster than oil-based polyurethane. Water-based polyurethane will dry in about an hour after its application on plastic. Water-based polyurethane will also cure in about 2 weeks after its application.

Oil-based polyurethane on the other hand will take longer. It can take up to 4 hours for oil-based polyurethane to dry and several days for it to cure after its application on plastic.

Another factor that determines how long polyurethane will take before drying on plastic is the number of polyurethane coats applied.

Like all other paint types, more coats of polyurethane will take longer to dry. That being said, you only need 3 coats of polyurethane on plastic. More than 3 coats will take days to dry and longer to cure. When it finally cures, the resulting finish will turn yellow.

Thicker coats of polyurethane will also take longer to dry.

The drying time of polyurethane is also affected by the condition of the plastic. Polyurethane applied on a filthy or greasy plastic surface will remain sticky or tacky for several weeks because the filth on the plastic will repel the polyurethane and stop it from drying quickly.

Other factors that influence polyurethane drying time on plastic includes room temperature, weather, and moisture content.

Related Read: Can Oil-Paint Be Used on Plastic?

How To Apply Polyurethane On Plastic (Made Easy)

Applying polyurethane to plastic isn’t difficult. But the smoothness of the finish depends on the prep work done before the paint was applied. Before we go on with this task, there is something you should know.

Tip: Never apply polyurethane on bare plastic. It’s possible to come across instructions indicating that polyurethane can be applied to plastic. These instructions should be disregarded.

Applying polyurethane on bare plastic will not produce a fine finish because the polyurethane will not stick to the plastic. This is due to the non-porous and slick nature of the plastic surface.

If you want to use polyurethane on plastic, it’s best to paint or prime the plastic first. This guide will teach you just how to do this. So when you are ready, let’s get to it. You are? Great. Let’s move on.

Tools Needed To Apply Polyurethane On Plastic

To apply polyurethane on plastic, you need the following tools and supplies:

  • Sandpaper or Sanding Block
  • A Can of Oil-based or Water-based Primer Clean Rags
  • Rubbing Alcohol or Dish Soap
  • Paintbrush or Spray Gun
  • Primer
  • Hair Dryer or Infrared Heater.

Here is a quick rundown of how to put polyurethane on plastic:

  1. Clean The Plastic With Rubbing Alcohol
  2. Sand The Plastic Surface
  3. Apply Two Coats of Primer On The Plastic
  4. Spray or Brush The Polyurethane On The Surface
  5. Dry The Polyurethane With a Hair Dryer.

Now, let’s get to work.

1. Clean The Plastic With Rubbing Alcohol

Clean The Plastic With Rubbing Alcohol

The first step is to wipe the plastic surface with rubbing alcohol. This is to remove grease and filth on the plastic. If the plastic isn’t clean, then paint coatings will not stick well to it.

To clean the plastic, douse a clean rag with rubbing alcohol. Then use the damp rag to wipe the surface repeatedly. If the surface is very dirty, you will need a soft brush to remove stubborn stains.

If you don’t have rubbing alcohol, you can mix some dish soap in water. Then use the soapy water to wash the plastic. Plastic surfaces are not porous so washing will not damage the plastic material.

2. Sand The Plastic

Sand The Plastic

The next step is to sand the plastic.

Sanding helps to abrade or scuff the plastic. When you sand, you create tiny pores in the plastic that the primer and polyurethane can bite into.

Sanding also helps to even out grain and create a smooth surface that makes the resulting finish fine and smooth.

To sand, use fine-grit sandpaper. Don’t use sandpaper less than 100-grit on plastic if not the plastic will be scarred. After sanding, vacuum, or dust the plastic surface to get rid of dust filters and dust nibs.

3. Apply Two Coats of Primer On The Plastic

Apply Two Coats of Primer On The Plastic

This step is very important. As stated earlier, polyurethane doesn’t adhere well to bare plastic.

Priming before applying the polyurethane will make the polyurethane adhere well to the plastic surface. This is because primer improves paint adhesion.

The choice of primer depends on the type of polyurethane you intend to use. If you plan on using water-based polyurethane, then you should use a water-based primer. Oil-based polyurethane will require an oil-based primer.

Tip: For Plastic surfaces, it’s better to use water-based polyurethane.

This is because water-based primers, paints, and clear coats adhere better to plastic than oil-based versions. Oil-based paints have natural or synthetic oils that will not stay on plastic.

To apply the primer, use a paintbrush. Apply two coats of primer on the plastic. Wait for the first primer coating to dry before applying the second coating. When both coats of primer have dried, you should…

4. Spray or Brush The Polyurethane On The Surface

Spray or Brush The Polyurethane On The Surface

Polyurethane can be applied using a paintbrush or a spray gun.

Tip: Spraying polyurethane produces better results but brushing is safer especially if you are using oil-based polyurethane.

Oil-based polyurethane has a high volume of strong chemicals and volatile organic compounds (VOCs).

These chemicals shouldn’t be inhaled in large quantities but if you spray the poly, you are at a higher risk of inhaling the chemicals.

Tip: If you want to spray polyurethane, it’s advised to open all doors and windows for adequate ventilation.

If possible, take the item to be sprayed outside and spray the polyurethane out in the open. You can also turn on fans in the room for proper airflow.

To apply polyurethane with a paintbrush, prepare the polyurethane first. Then dip the paintbrush in the container.

After, press the paintbrush against the inside of the polyurethane container to remove excess polyurethane. This is because it’s better to apply polyurethane in light coats so it’s easier to control and dry faster.

When you have removed the excess polyurethane, apply the polyurethane in light strokes on the plastic. 3 light coats of polyurethane are enough.

To use a spray gun, prime the sprayer and prep the polyurethane. Then pull the trigger and test the paint flow on cardboard before moving to the plastic. Also, 3 coats of polyurethane are enough.

Tip: You don’t need to sand between coats of polyurethane as long as you primed the surface before applying the polyurethane.

Related Read: Can You Use a Rag To Apply Polyurethane?

5. Dry The Polyurethane With a Hair Dryer

Dry The Polyurethane With a Hair Dryer

After applying the final coat of polyurethane, plug in a hairdryer and move it across the polyurethane coating for a few minutes. This step is optional but it makes the polyurethane dry faster.

When you have completed all these steps, you have successfully applied polyurethane over plastic.

Can You Apply Polyurethane On Outdoor Plastic?

You can apply polyurethane on outdoor plastic as long as the plastic surface is sanded and primed first.

Also, you should use exterior polyurethane for outdoor plastic rather than interior polyurethane.

This is because exterior polyurethane is designed with additives that make it less prone to outdoor weather, moisture, dust, and the sun’s ultraviolet rays.

Polyurethane is an ultra-thick finish that is scratch, corrosion, and water-resistant. Using polyurethane on outdoor plastic will preserve the finish of the plastic and also make the plastic last longer.

Outdoor plastic surfaces are exposed to increased moisture, snow, dust, and heat. All of these factors will cause the plastic surface to deteriorate and damage. But polyurethane prevents this.

This is because polyurethane when fully cured forms an ultra-thick sheen that prevents these said factors from damaging the plastic surface.

Interior plastic that will witness heavy use like chairs, tables, and drawers can also benefit from being sealed with polyurethane.

Related Read: Can You Apply Exterior Polyurethane Over Interior Polyurethane?

Do I Need To Prime Plastic Before Applying Polyurethane?

You need to prime plastic before applying polyurethane. This is because plastic surfaces are non-porous and slick. This makes it very difficult for polyurethane to adhere well to the bare plastic surface.

Priming the plastic surface before applying polyurethane will improve the adhesive qualities of the plastic and make the polyurethane stick to it well.

Priming also helps to achieve a smooth and durable polyurethane finish on plastic.

Plastic surfaces are tough to paint over without priming first. This is because the surface has no grain or fiber for the polyurethane to sink and bite into. This prevents proper paint bonding.

But with two coats of primer, this problem is averted. Paint primers are designed to improve paint bonding, so priming plastic before applying polyurethane is very important and necessary for a smooth and durable finish.

The only scenario where you might not prime plastic first before applying polyurethane is if the plastic has been previously painted with latex paint. If this happens, you can get away with not priming the plastic as long as you sand the latex finish on the plastic lightly. Then apply the polyurethane.

Usually, polyurethane will stick just fine on latex paint but latex paint shouldn’t be used as a substitute for primer.

Related Read: Can You Apply Polyurethane on Metal?

Final Words

Overall, applying polyurethane to plastic is difficult due to poor adhesion. However you can still apply the polyurethane if you clean, sand, and prime the plastic before applying the polyurethane.

Remember never to apply polyurethane on bare plastic. The finish wouldn’t be as expected.

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