Polyurethane On Metal: Can You Use it And How?

You can use polyurethane over wooden surfaces to protect them. But, can you use polyurethane on metal?

You can apply polyurethane on metal surfaces, but you must apply a primer (or paint) before applying polyurethane. Polyurethane will protect metal surfaces from rust and gives the metal a clear finish.

But that’s not all. There’s more to know about using polyurethane paint for metal, and this post explains everything. 

Polyurethane Prevents Rust on Metal

Polyurethane prevents rust on metal. When dry, polyurethane forms a thick clear finish over metal. This thick finish makes the metal rust-resistant and repels moisture and water.

Rust damages metal faster than anything else. So, when choosing a finish for metal, you must pick one that’s rust-resistant. Polyurethane is one of the best finishes for metal. The thick and glossy layer of polyurethane will act as a protective layer to prevent rust. 

Rust is produced when the metal surface is exposed to water, air, or an acidic substance. But, since polyurethane forms a water-resistance layer over metal, the metal won’t rust. 

However, you shouldn’t use polyurethane for outdoor metals. Though polyurethane is a strong finish, it is more suited for indoor use. The weather elements will affect the polyurethane finish making it less durable. With time, the polyurethane will tarnish and expose the outdoor metal to elements that can produce rust. 

For outdoor metal, you must use exterior polyurethane. 

Related Read: How To Paint Concrete With Polyurethane Paint?

How To Apply Polyurethane On Metal?

Applying polyurethane over metal is an easy process. However, you need the right guidance and tools. Luckily, we have prepared a step-by-step guide for you. 

The guide below is for applying polyurethane on bare metal. If you want to apply polyurethane on painted metal, sand the paint with fine-grit sandpaper, clean the surface and then apply the polyurethane finish. 

1. Clean The Metal Surface

Clean The Metal Surface

Before applying polyurethane, the metal must be clean and free of dust and rust. If the metal surface is dirty, the dirt will show since polyurethane is a clear coat (it has no color).

To remove dirt from metal:

  1. Wash the surface with mild detergent, water, and a sponge.
  2. Apply the mild detergent over the metal surface, and use the sponge to remove the dust.
  3. After you are done, rinse the metal with clean water. 

To remove rust from metal, spray white vinegar over the metal surface, wait a few minutes, and then remove the white vinegar residue with clean water. You can use rubbing alcohol or acetic acid as an alternative to white vinegar. 

After removing dust and rust from the metal surface, let the metal dry. After, you can use a damp rag to remove any leftover dirt or grime. 

2. Sand The Metal Surface

Sand The Metal Surface

Before applying polyurethane, you must sand the metal surface. Sanding will remove any imperfections or bumps the metal surface may have. 

To sand the metal, use sandpaper designed for metal surfaces. Diamond sandpaper is a good choice. You can either dry sand the metal by rubbing the sandpaper against the metal surface repeatedly. Or, wet sand the metal by using water and waterproof sandpaper. 

Metal blasting (or sandblasting) is an alternative to sanding. The sandblaster will spray sand at high pressure (and speed) to the metal surface. The metal blasting will remove imperfections and bumps from the metal. However, metal blasting is recommended for outdoor large-scale metal surfaces. So, if you have an indoor metal, it’s better to sand it. 

After sanding or blasting the metal surface, remove the dust on the metal using a vacuum.

3. Prime The Metal Surface

Prime The Metal Surface

Except for sanding, you must also prime the surface before polyurethane. That’s because polyurethane doesn’t stick well to bare metal surfaces, so priming helps the polyurethane adhere better. However, if the metal is already painted (or stained), priming isn’t necessary, but it can help. 

To prime metal for polyurethane, use self-etching primer or epoxy primer. Using a paintbrush apply 2 coats of primer to the metal surface. If you use an epoxy primer, ensure that it is designed for metal surfaces.  

Related Read: Can You Apply Polyurethane With a Rag?

4. Mix The Polyurethane

Mix The Polyurethane

Since polyurethane is a clear coat finish, you must mix it before using it. To mix polyurethane, follow the manufacturer’s instructions. For most polyurethane brands, you must stir the polyurethane (with a turning stick) for a few minutes. This ensures consistent flow. 

To stir the poly, use a turning 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.

For water-based polyurethane, you don’t need to use too much force while stirring the polyurethane. But, since oil-based polyurethane is thicker, you must use a bit more force while stirring. It’s recommended to use a paint mixer for oil-based polyurethane. 

Related Read: How To Tint Polyurethane Paint?

5. Apply The Polyurethane

Apply The Polyurethane Finish On The Metal

To apply polyurethane on metal, use a paintbrush or paint sprayer. You shouldn’t apply polyurethane with a roller to metal surfaces because the finish will be riddled with bubbles. 

A synthetic bristled brush is the best type of paintbrush to apply polyurethane over metal. The bristles of this paintbrush are made of synthetic materials such as nylon and polyester. This allows the brush to apply a smooth layer of polyurethane. To apply polyurethane with a paint sprayer, use an airless paint sprayer.

You must apply thin coats of polyurethane. So, use light brush strokes for paintbrushes or small nozzles for paint sprayers. 

6. Sand Between Coats

Sand Between Coats

While drying, the polyurethane is affected by dust. So, you must sand between coats of polyurethane to remove the dust and grains that have settled over the polyurethane. You should sand after the polyurethane has fully dried.

You shouldn’t use rough sandpaper as you can remove the polyurethane. Instead, use fine-grit sandpaper (400-grit). Also, don’t apply too much pressure while sanding; just swipe the sandpaper over the polyurethane.

Once the first coat of polyurethane dries and you sand it, apply the next one. Wait for the second coat to dry, sand it, and then apply the third coat. You need 3 coats of polyurethane on metal. 

You shouldn’t sand the last coat of polyurethane. If you do, the finish will end up cloudy or scarred. 

7. Leave The Polyurethane Paint To Dry

Leave The Polyurethane Paint To Dry

After you have applied the final coat of polyurethane on metal, leave the finish to dry. Water-based polyurethane takes 12-24 hours to dry, it takes oil-based polyurethane takes 24-48 hours to dry. 

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

Polyurethane Makes Metal Water-Resistant

Polyurethane makes the metal surface water-resistant but not waterproof. Though polyurethane is thick and strong, it still has some porosity. The more porous the polyurethane is, the more water it allows to pass through.

The thick layer of polyurethane is water-resistant and prevents metal water absorption. This means polyurethane prevents water from reaching the metal surface underneath. However, polyurethane isn’t waterproof

The water-resistance level of polyurethane depends on the porous level of the polyurethane. So, while choosing a polyurethane brand/type for metal, check the water-resistant level on the container. The more water-resistant the polyurethane is, the more rust-resistant it will be. 

Polyurethane isn’t waterproof, but it protects the metal from water damage for at least 5 years.

Does Polyurethane Stick To Aluminum?

Polyurethane doesn’t stick to aluminum surfaces without sanding or priming. So, you must sand and prime the aluminum before applying polyurethane.

The aluminum surface is slick, so the polyurethane has nothing to stick to. So, to use polyurethane on aluminum, you must prep the surface first.

Here’s how to do it:

  1. Wash and clean the aluminum surface to remove dirt or grime.
  2. Sand the aluminum with medium-grit sandpaper and then with fine-grit sandpaper.
  3. Apply self-etching primer. 
  4. Sand the primer surface once it dries.
  5. Apply polyurethane over aluminum. 

Using Polyurethane On Painted Metal

You can use polyurethane over painted metal to protect the paint and metal from scratches, water, and rust. But, you must sand the painted surface before applying the polyurethane. 

To sand painted metal, use fine-grit sandpaper (240-grit) and above. After sanding, use a shop vac to vacuum the dust produced by sanding. If you don’t sand painted metal, the polyurethane won’t adhere well to the surface, and the finish will be uneven.

The surface of the painted metal is riddled with dust particles that can prevent the polyurethane from sticking. So, when you sand, you remove the dust and scratch the paint a bit to increase the polyurethane adhesion. 

A coat or two of polyurethane will protect the metal from scratching, handling, water, and cool temperature. Without a sealer (such as polyurethane), the metal surface will get damaged by weather elements. 

Final Words

You can use polyurethane for metal, aluminum, steel, and iron surfaces. But, you must clean, sand, and prime the surface before painting. This is because polyurethane doesn’t stick well to bare metal. 

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