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

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

You can apply polyurethane over a plastic surface, but you must prep the surface first. That’s because plastic is non-porous, so the sealant won’t adhere well to it. 

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

Compatibility

Polyurethane does stick to bare plastic, but the adhesion won’t be good. That’s because plastic is a non-porous material with a slick and impermeable nature, so it’s hard for the sealant to stick over it. 

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

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

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

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

Priming Helps

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

Primers are formulated with extra bonding additives that help them to stick to different surfaces. Since the primer sticks to plastic, it will create a base layer and improve the adhesion of the sealer. Polyurethane will stick over the primer coating, and not the surface underneath it. 

However, a primer will cover the original color of the surface. And, since polyurethane is a clear coat, the finish will have the same color as the primer. Therefore, if you want the surface 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 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 its surface using rubbing alcohol to remove grease and filth. If the surface isn’t clean, the finish will remain tacky for days and won’t stick. 

To clean it:

  1. Douse a clean rag with rubbing alcohol. 
  2. Use the dampened rag to wipe the surface. 
  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 them.

2. Sand The Plastic

Sand The Plastic

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

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

3. Apply Primer

Apply Two Coats of Primer On The Plastic

Optionally, you can prime the plastic before sealing it. A primer will stick to plastic and create a base coat for the sealant to stick to. Primers are formulated with extra bonding additives that help them to stick to different surfaces.

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

Apply 2-3 coats of primer 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. 

The primer will cover the original color of the surface, and since polyurethane is a clear coat, the surface will have the same color shade as the primer. So, if you want to keep the original color of the surface, you shouldn’t prime it. Or, you must apply 1-2 coats of paint or use a colored primer.

4. Apply The Polyurethane

Spray or Brush The Polyurethane On The Surface

Once the plastic is sanded and primed, apply the polyurethane. To apply it, use a paintbrush or a spray gun. You can also use a polyurethane spray-on.

If you use oil-based polyurethane, you must thin it before spraying since it is too thick to get expelled from the sprayer nozzle. To thin it, use mineral spirits (ratio 4:1). If you use a paintbrush or use the water-based type, you don’t have to thin it. 

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

5. Dry The Finish

Dry The Polyurethane With a Hair Dryer

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

However, use a hairdryer only for 2-3 minutes. If you dry the finish too fast, it 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 finish dry naturally.

Polyurethane Dry Time on Plastic

It takes water-based polyurethane 8 hours to dry enough for a re-coat and 24 hours to cure, while oil-based poly takes 24 hours to dry between coats and 48-72 hours to cure.

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

For the sealant to cure, it 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 it doesn’t penetrate its surface deeply, meaning the evaporation process starts immediately after you apply the sealant. Since the evaporation process starts sooner, the finish dries and oxidizes faster.

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

Other factors that influence the drying time are room temperature, weather, moisture content, number of coats, and thickness.

Polyurethane On Outdoor Plastic

You can use exterior polyurethane on outdoor plastic as it is formulated with UV resistance additives and is durable for outdoor weather elements.

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

When dry, exterior polyurethane forms a glossy layer resistant to water, moisture, scratches, and UV rays that protects the surface underneath from all these elements. 

However, you must clean, sand, and prime the outdoor surface before sealing it.

Final Words

You can apply polyurethane over plastic, but you must sand and prime its surface first. If you don’t. the sealant won’t stick over to the slick and bare surface of the plastic.

Tony Adams
Tony Adams

Tony is a professional painter and an author of DIY Geeks. Tony has completed over 1,000 painting projects for his clients. It's safe to say he knows what he Is talking about,

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