Can You Use Polyurethane Paint on Metal?

I had a chipped metal frame and a can of polyurethane finish but I wasn’t sure if I could use the polyurethane on metal. So I made some research and here is what I found out.

You can use polyurethane finish on metal surfaces but you will need to prime or blast the metal before applying polyurethane. Polyurethane is an ideal finish for metal surfaces because it protects the metal, prevents rust, and gives a clear finish on the metal.

But that’s not all. This post reveals more about using polyurethane on metal including how to apply polyurethane on metal correctly. So keep reading to find out more.

Will Polyurethane Prevent Rust On Metal?

Yes, polyurethane paint prevents rust on metal. That’s because polyurethane when applied correctly dries to form a tough and clear film on the metal. The tough film makes the metal rust-resistant because it prevents harsh weather and water from coming in contact with the metal.

When it comes to choosing a finish to use on metal, one thing to always consider is if the finish can prevent rust. This is because rust damages metal quicker than anything else. If the finish can’t prevent rust, then the metal surface wouldn’t last very long.

Polyurethane is one of the best finishes that prevents rust on metal surfaces. This is because polyurethane when fully cured forms an ultra-thick and clear film on the surface of the metal. The thick and clear film on the metal surface acts as a protective layer that prevents rust on the metal.

For metal to rust, the metal surface has to be exposed to water, air, and or an acidic substance. In most cases, water is enough to cause rust on metal and polyurethane when dry is water-resistant. So the polyurethane finish will prevent rust on the metal.

However, poly isn’t advised for use on outdoor metal. Though polyurethane is a strong finish, it is more ideal for indoor use.

If you use polyurethane on outdoor metal, the polyurethane will still prevent rust but only for a while. This is because the elements (or weather) will negatively affect the polyurethane finish making it less durable. With time, the polyurethane will tarnish and expose the outdoor metal to the elements which will in turn cause rust.

Let’s check out how to apply polyurethane to metal.

Related Read: How To Paint Concrete With Polyurethane Paint?

How To Apply Polyurethane On Metal (Made Easy)

Applying polyurethane on metal is a fairly easy process. To apply the finish, you need to:

  • Clean The Metal Surface
  • Sand Or Blast The Metal Surface
  • Prime The Metal Surface
  • Prep And Mix The Polyurethane Finish
  • Apply The Polyurethane Using A Paint Sprayer Or A Paintbrush
  • Sand Between Coats
  • Leave The Polyurethane To Dry

Now let’s check out these steps in detail.

As an expert tip, the steps below are for applying polyurethane on bare metal. If you want to apply polyurethane on painted metal, you just have to sand the paint lightly with fine-grit sandpaper, clean the surface, and then apply the polyurethane finish.

Now, let’s get back to work.

1. Clean The Metal Surface

Clean The Metal Surface

The first step is to ensure the metal surface to be painted is clean and free of any rust or dirt material. If the metal surface is filthy, you will need to wash it first. You can wash the metal surface with mild detergent, water, and a sponge. You can also strip the metal surface by using a wire sponge or brush.

If you notice rust on the metal surface, you can get rid of it by spraying white vinegar on the metal surface. If you don’t have white vinegar, you can use rubbing alcohol or any other acetic acid to get rid of the rust.

After washing the metal, you need to let it dry. Then you can use a damp rag to wipe the metal surface to reveal any leftover dirt or grime. When the metal is clean, you can go ahead to the next step.

2. Sand The Metal Surface

Sand The Metal Surface

The next step is a continuation of prep work. The metal surface to be painted has to be sanded before applying the polyurethane so the metal is smooth and easier to paint.

To sand the metal, you will need sandpaper that is designed for metal surfaces. The best bet is to go with diamond sandpaper. You can either dry sand by rubbing the sandpaper against the metal surface repeatedly. Or you can wet sand by using water and waterproof sandpaper.

Another alternative to sanding is to blast the metal. Blasting is also known as sandblasting. This is a process of using a sandblaster to spray sand at high pressure on the metal surface.

Sandblasting is often used on large-scale tasks and it’s not advised for indoor use. So if you plan on painting indoor metal or metal at home, it’s better to sand manually with sandpaper. Sandblasting should be left for much larger and commercial projects.

After sanding or blasting the metal surface, get rid of the dust on the metal with a vacuum before moving on to the next step.

Related Read: What Happens If You Don’t Sand Polyurethane Paint?

3. Prime The Metal Surface

Prime The Metal Surface

After sanding or blasting the metal, you will need to prime the metal too. This is because polyurethane doesn’t bond very well to metal surfaces. Priming the metal before painting will prep the metal surface so the polyurethane can sit well.

To prime metal for polyurethane, you will need an etching primer or epoxy primer. Especially if you are applying the polyurethane on bare metal.

However, if the metal has been painted or stained, you wouldn’t need to prime the surface. You just need to sand lightly with fine-grit sandpaper to allow the polyurethane to sit well on the paint.

Ensure to use an epoxy primer that is designed for use on metal. Usually, a coat or two of epoxy or etching primer is enough to prime the metal for polyurethane.

Related Read: Can You Apply Polyurethane With a Rag?

4. Mix The Polyurethane

Mix The Polyurethane

Polyurethane is a clear coat finish and most clear coat finishes usually need to be mixed before using them.

Follow the manufacturer’s instructions while mixing the polyurethane finish. For most polyurethane brands, you just need to stir the polyurethane in the container for a few minutes. That will ensure consistency and uniformity.

To stir the poly, you can use a clean stick or a paint mixer. To stir manually, put a clean stick or rod in the paint container and stir the paint with gentle swirls.

If you are using water-based polyurethane, you wouldn’t need much force to stir the polyurethane in the container. But if you are using an oil-based poly, you will need to use a bit of force because oil-based polyurethane is thick.

Your best bet for mixing oil-based polyurethane is to use a paint mixer.

Related Read: How To Tint Polyurethane Paint?

5. Apply The Polyurethane Finish On The Metal

Apply The Polyurethane Finish On The Metal

Polyurethane can be applied by using a paintbrush or a paint sprayer. You shouldn’t use a paint roller to apply polyurethane because the finish will be riddled with bubbles.

The best type of paintbrush to use when applying polyurethane finish to metal is a synthetic bristled brush. The bristles are usually made from synthetic materials like nylon and polyester. That makes the brush ideal for applying polyurethane. You can also use an airless paint sprayer to apply the polyurethane.

To apply the poly, ensure to start with thin coats. This means you should use light brush strokes if you are using a paintbrush or a small nozzle if you are using a paint sprayer.

After applying the first coat of polyurethane, leave it to dry.

Related Read: Can You Put Too Many Coats Of Polyurethane?

6. Sand Between Coats

Sand Between Coats

Polyurethane like most finishes doesn’t have the best adhesive qualities. The finish is also easily affected by dust while it is drying. So you will need to sand between coats of polyurethane to get rid of dust and grains that settle on the poly while it’s drying.

You don’t need to sand aggressively or you will ruin the coat. You need fine-grit sandpaper like 400-grit and above. You also don’t need much pressure while sanding. You just need to swipe the polyurethane with sandpaper.

When the first coat has dried and is lightly sanded, you can apply a second coat of polyurethane. After applying the second coat, leave it to dry and then sand lightly again before applying the third coat.

You need 3 coats of polyurethane on the metal. The last coat of polyurethane shouldn’t be sanded. If you sand the last coat, you will end up with a cloudy or scarred finish.

7. Leave The Polyurethane Paint To Dry

Leave The Polyurethane Paint To Dry

When you have applied enough coats of polyurethane on the metal, leave it to dry. It takes 3-24 hours for water-based polyurethane to fully cure. Oil-based polyurethane will take longer.

When the polyurethane has cured, you will have an ultra-thick and clear coating on the metal surface.

Related Read: Can You Paint Metal With Chalk Paint?

Does Polyurethane Waterproof Metal?

Polyurethane finish makes metal water-resistant but not completely waterproof. This is because polyurethane finish though thick and strong still has some degree of porosity. The more porous the polyurethane is, the more likely it is to allow water to pass through.

Polyurethane dries to form a thick layer of clear coat on metal. This thick layer of clear coat is water-resistant and prevents water absorption by the metal. This means the polyurethane prevents water from reaching the metal underneath. But polyurethane is not a hundred percent waterproof.

The degree to which a polyurethane finish can prevent water from reaching the metal underneath depends on how porous the polyurethane finish is. So if you are picking a polyurethane finish for metal, always check the water-resistant qualities on the container to know which will be ideal to use on metal. The more water-resistant polyurethane is, the more rust-resistant it will be.

So polyurethane makes metal water-resistant but the water resistance of the metal depends largely on the type and brand of polyurethane you are using on the metal.

This doesn’t however mean that metal will be affected by water if polyurethane is used on it. Even though it’s not a hundred percent waterproof. Polyurethane will protect the metal from water for at least 5 years.

Another thing you should know is if polyurethane sticks to aluminum. So does it? Let’s find out.

Related Read: Can You Use Wood Paint on Metal?

Does Polyurethane Stick To Aluminum?

Polyurethane finish doesn’t stick to aluminum surfaces. But the polyurethane will stick if the aluminum surface is sanded and primed with a self-etching primer before applying the polyurethane.

Polyurethane doesn’t stick well to aluminum because the aluminum surface is slick and the polyurethane will have nothing to bite into. So adhesion of polyurethane on aluminum is very difficult. However, it’s not impossible.

To use polyurethane on aluminum, you need to do some prep work.

First, you will need to wash and clean the aluminum surface to get rid of any dirt or grime. Next, you will need to sand the aluminum surface starting with medium-grit sandpaper and then fine-grit sandpaper.

After sanding the aluminum, you will also need to apply a self-etching primer. Then when the primer dries, you will need to sand lightly again before applying the polyurethane finish.

After applying the polyurethane finish, you can also apply a sealant but this is usually not needed. The polyurethane finish is good enough without a sealant.

Applying polyurethane on bare aluminum will be very difficult and messy due to the nature of the aluminum surface. But after some prep work, you can make it happen.

Related Read: Should You Use Latex Paint on Metal?

Can You Use Polyurethane On Painted Metal?

You can use polyurethane on painted metal to give the metal increased durability and protection against scratches and rust. But you need to sand the painted surface before applying the polyurethane so it can adhere well to the surface.

Applying a coat or two of polyurethane over a painted metal surface is a great way to protect the finish of the metal.

Ordinarily, painted metal would come out nice. But, it wouldn’t last long because the paint would be exposed to scratches, handling, water, and cool temperature. All of these can ruin the painted metal.

So to give the painted metal increased durability and water resistance, you can apply a coat or two of polyurethane finish over the painted surface. But you will need to do a bit of sanding first.

You will need to sand the painted metal with fine-grit sandpaper like 240-grit and above. After sanding, you should use a shop vac to vacuum the sanded metal, and then you can go ahead to apply polyurethane on the painted metal.

Without sanding, the polyurethane wouldn’t adhere well to the surface and the finish will be ugly and uneven. This is because the painted metal will be riddled with dust particles that will negatively affect the polyurethane application on the metal.

So when you sand, you are getting rid of the dust and scarring the paint a bit to allow the polyurethane to sit on the paint well.

Related Read: Does Polyurethane Paint Expire?

Final Words

Overall, you can use polyurethane finish on metal surfaces including aluminum, steel, and iron. But you need to do some prep work including cleaning, sanding, and priming. This is because polyurethane wouldn’t stick well to bare metal but it would if the metal is fully prepped.

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