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 over metal surfaces, but you must apply a primer (or paint) before applying it. Polyurethane will protect metal surfaces from rust and gives them a clear finish.

Does it Prevent Rust?

Polyurethane prevents rust on metal because it forms a clear moisture-resistant finish that prevents moisture from affecting surfaces. Since moisture can’t affect its surface, rust won’t be produced. 

Rust damages metal faster than anything else. So, when choosing a finish for it, you must pick one that’s rust-resistant. Polyurethane is one of the best finishes as its coating acts 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 on the surface, the metal won’t rust. 

However, you shouldn’t use normal polyurethane for outdoor metals because weather elements will affect its finish making it less durable. With time, its finish will tarnish and expose the surface 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. 

1. Clean The Metal Surface

The metal surface must be clean and free of dust and rust. If it’s dirty, the dirt will show once the sealant dries since polyurethane is a clear coat (it has no color).

To clean it:

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

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

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

2. Sand The Metal Surface

Sanding will remove any imperfections or bumps the surface may have. 

To sand the metal, use sandpaper designed for metal surfaces. Diamond sandpaper is a good choice. You can either dry sand it by rubbing the sandpaper against its surface repeatedly. Or, wet sand it 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) on the surface. The blasting will remove imperfections and bumps from the surface. However, metal blasting is recommended for outdoor large-scale surfaces. 

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

3. Prime The Metal Surface

To get better adhesion, apply one coat of primer over the metal before sealing it. Polyurethane doesn’t stick well to bare metal, so having an undercoat (primer or paint) improves the adhesion. 

To prime metal, use self-etching primer or epoxy primer. Using a paintbrush apply 2 coats of primer. 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

Stir (or shake) the polyurethane in the container to get a consistent flow. You can use a turning stick or paint mixer for this. 

For water-based polyurethane, you don’t need to use too much force while stirring it. 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 it.

Related Read: How To Tint Polyurethane Paint?

5. Apply The Polyurethane

To apply polyurethane, use a paintbrush or paint sprayer. Use a synthetic bristled brush as its bristles as made of synthetic materials and apply polyurethane smoothly and evenly. You shouldn’t apply it with a roller to metal surfaces because the finish will be riddled with bubbles. 

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

6. Sand Between Coats

While drying, the sealant is affected by dust and dirt. So, you must sand between coats to remove the dust and dirt that have settled over the coating. You should only sand after the coating has dried. 

To sand between coats, use fine-grit sandpaper. This sandpaper will only remove the imperfections and bumps from the coating, and not the entire finish. Avoid using coarse or medium-grit sandpaper as it can damage it. 

Once the first coat dries, sand it, and apply the next one. You need three coats of polyurethane over non-porous surfaces. Don’t sand the final coat. 

Does it Protect The Metal?

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

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

Its water-resistance levels depend on its porous levels. So, while choosing a polyurethane brand/type, check the water-resistant level on the container. The more water-resistant the sealant is, the more rust-resistant it will be. 

Does Polyurethane Stick To Aluminum?

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

The aluminum surface is slick, so the sealant has nothing to stick to. So, to use it 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 it 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 surface from scratches, water, and rust. But, you must sand the painted surface before applying the sealant. 

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 it, the sealant won’t adhere well to the surface, and the finish will be uneven.

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

A coat or two of polyurethane will protect the paint from scratching, handling, water, and cool temperature. Without a sealer, 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 sealing them.