Polyurethane on Plastic (Can You Do it & How-To)

Polyurethane is a clear coat sealant that protects the surface underneath from moisture, water, and scratches. So, can you apply polyurethane over plastic?

You can use a polyurethane coating for plastic, but you must prep the plastic first. That’s because plastic is non-porous, and polyurethane won’t adhere well to it. So, you must prep the plastic first to help polyurethane stick better.

To prep a plastic surface, you must clean, sand, and apply a primer. However, applying a primer is optional. 

Does Polyurethane Stick To Plastic?

Polyurethane doesn’t stick to bare plastic because plastic surfaces are non-porous. Plastic’s slick and impermeable nature makes it impossible for polyurethane to penetrate and stick to it.

Polyurethane is a topical sealant that doesn’t need to penetrate the surface to stick. However, plastic has a slick surface, and polyurethane will slide off the plastic surface when applied. So, polyurethane needs to penetrate (bite into) the plastic surface a bit to stick.

You must sand or apply primer to help polyurethane stick to plastic. Sanding will create tiny ridges (or scratches) on plastic surfaces for polyurethane to soak into and stick. 

A primer will create a smooth base coat for polyurethane to stick to. So, polyurethane will stick to the primer coating instead of the plastic surface. Since primer has a dry textured finish, polyurethane will stick to it. 

Polyurethane can also stick to painted plastic. That’s because polyurethane will stick (bite into) the paint coating and not the slick plastic surface. 

Priming Helps

You must apply a primer to plastic before applying polyurethane. That’s because plastic is non-porous and slick, and polyurethane won’t adhere well to bare plastic.

Primers are formulated with extra bonding additives that help them to stick to plastic. Since the primer sticks to plastic, it will create a base layer and improve the bonding between polyurethane and plastic. That’s because polyurethane will stick to the primer layer, not the slick plastic surface.

However, a primer will cover the color shade of the plastic. And, since polyurethane is a clear coat, the finish will have the same color as the primer. Therefore, if you want the plastic to have a different color, you shouldn’t prime it. To avoid this, you can apply one coat of paint after the primer dries.

How To Apply Polyurethane On Plastic?

Applying polyurethane to plastic isn’t hard, but the finish results depend on how well you prep the plastic surface. Also, you shouldn’t apply polyurethane to bare plastic because it won’t stick. Instead, you must sand and prime the plastic.

Here are the tools you need:

  • Sandpaper or sanding nlock.
  • Oil or water-based primer.
  • Rubbing Alcohol or Dish Soap.
  • Paintbrush or Spray Gun.
  • Hair Dryer.

1. Clean The Plastic

Clean The Plastic With Rubbing Alcohol

First, clean the plastic surface using rubbing alcohol. This will remove the grease and filth from the plastic. If plastic isn’t clean, polyurethane will remain tacky for days and won’t stick. 

To clean a plastic:

  1. Douse a clean rag with rubbing alcohol. 
  2. Use the dampened rag to wipe the plastic. 
  3. Use a soft brush to remove stubborn stains. 
  4. Rinse the brush to remove rubbing alcohol residue. 

You can use soapy dishwater as an alternative to rubbing alcohol. Plastic surfaces are not porous, so washing won’t damage the plastic material.

2. Sand The Plastic

Sand The Plastic

You must sand the plastic after you clean it. Sanding creates tiny pores in the plastic that the polyurethane can bite into. It also evens out the grain and creates a smooth surface for the polyurethane to stick to. 

To sand plastic, use fine-grit sandpaper. You shouldn’t use sandpaper with less than 100-grit as it can scar the plastic surface. 

3. Apply Primer

Apply Two Coats of Primer On The Plastic

Optionally, you can prime the plastic before applying plastic. A primer will stick to plastic and create a base coat for polyurethane to stick to. Primers are formulated with extra bonding additives that help the primer to stick to plastic.

You can use either water- or oil-based primer for plastic. The choice also depends on the polyurethane type you will use. However, water-based primer sticks better to plastic. 

Apply 2-3 coats of primer to plastic using a paintbrush. You must wait until one coat of primer dries before applying the next one. Optionally, you can sand between coats of primer, including the final coat. 

Primer will cover the true color of the plastic. And since polyurethane is a clear coat, the plastic will have the primer color shade. So, if you want to keep the true color of plastic, you shouldn’t prime it. Or, you must apply 1-2 coats of paint before applying the polyurethane. 

4. Apply The Polyurethane

Spray or Brush The Polyurethane On The Surface

Once the plastic is sanded and primed, apply the polyurethane. To apply polyurethane to plastic, use a paintbrush or a spray gun. You can also use a polyurethane spray for plastic (spray-on poly). 

You must thin oil-based polyurethane before spraying it. That’s because oil-based polyurethane is too thick to get expelled from the spray gun nozzle. 

To thin oil-based polyurethane, use mineral spirits or turpentine in a ratio of 1:4 ( 1 part paint thinner, 4 parts polyurethane). You don’t have to thin water-based poly. But, if you use a paintbrush, you don’t have to thin oil-based polyurethane. 

Apply 3-4 coats of polyurethane over plastic. Wait for each coat to dry, sand the polyurethane coat, and then apply the next coat. You shouldn’t sand the final coat of polyurethane. 

5. Dry The Polyurethane

Dry The Polyurethane With a Hair Dryer

You can use a hairdryer to speed up the dry time of polyurethane. The hairdryer will increase the evaporation process, and the polyurethane will dry faster.

However, use a hairdryer only for 2-3 minutes. If you dry the polyurethane too fast, the finish will crack. That’s because the paint particles won’t have enough time to harden and bond naturally. So, after 2-3 minutes, let the polyurethane dry without a hairdryer. 

Polyurethane Dry Time on Plastic

Water-based polyurethane dries enough for a re-coat within 8 hours. While oil-based polyurethane takes 24 hours to dry enough for a re-coat on plastic. Water-based polyurethane needs 24 hours to cure, while oil-based polyurethane needs 48-72 hours.

For polyurethane to dry, the paint solvent (water or oil) must evaporate. Since water evaporates faster than oil, water-based polyurethane dries faster than oil-based polyurethane. 

For polyurethane to cure, the sealant must oxidize. This means the paint particles must harden and bond together. The oxidization process happens after evaporation. 

Polyurethane dries faster on plastic than on any other surface. That’s because polyurethane doesn’t penetrate the plastic surface, meaning the evaporation process starts immediately after you apply the polyurethane. Since the evaporation process starts sooner, the polyurethane dries and oxidizes faster on plastic. 

The drying time of polyurethane also depends on the plastic conditions. For example, if you apply polyurethane over a filthy or dusty plastic surface, it will remain tacky for several days before it dries. So, you must always clean the plastic before applying polyurethane.

Other factors influencing polyurethane drying time on plastic are room temperature, weather, moisture content, number of coats, and thickness.

Polyurethane On Outdoor Plastic

You can use exterior polyurethane on outdoor plastic. Exterior polyurethane is formulated with UV resistance additives and is more durable than normal polyurethane. So, exterior polyurethane will protect outdoor plastic for longer than normal polyurethane. 

Outdoor plastic surfaces are exposed to rain, water, dust, and heat. All of these factors will damage and discolor the plastic surface. So, you must seal the plastic with an exterior sealant, such as polyurethane, to prevent it from getting damaged.

When dry, exterior polyurethane forms a glossy layer resistant to water, moisture, scratches, and UV rays. The exterior polyurethane will protect the plastic underneath from all of these things.

However, you must clean, sand, and prime the outdoor plastic before applying polyurethane. 

Final Words

You can apply polyurethane over plastic, but you must sand and prime the plastic before. That’s because polyurethane won’t stick to bare plastic due to the slick surface the plastic has. 

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