Polyurethane is a versatile synthetic polymer coating commonly used to protect and seal wood surfaces. But can you use it on metal too? The short answer is yes, polyurethane can stick to metal if you follow the right steps.
This comprehensive guide will cover everything you need to know about using polyurethane on metal surfaces. We will look at:
- Why you might want to use polyurethane on metal
- Properly preparing the metal surface
- Step-by-step application process
- Polyurethane’s effectiveness against rust
- Special cases like aluminum and painted metal
- Frequently asked questions
So whether you’re looking to seal a metal tabletop, waterproof some metal furniture, or prevent rust on metal tools, this guide will equip you with the knowledge to do it right. Let’s get started!
Why Use Polyurethane on Metal?
Polyurethane is valued for its hardwearing and protective abilities. When applied to wood, it seals the surface, making it more resistant to water, scratches, and everyday wear and tear.
So why not use it to give your metal surfaces the same protective finish? Here are some key benefits of using polyurethane on metal:
1. Prevent Rust and Corrosion
The biggest reason to use polyurethane on metal is to prevent rust. As you may know, bare metal can corrode and rust when exposed to moisture. The polyurethane coating forms a protective seal that prevents moisture from reaching the metal.
2. Add a Clear, Glossy Finish
Polyurethane leaves behind a crystal clear, glossy finish that enhances the natural look of metal. It brings out a smooth polished look.
3. Make Metal Furniture More Durable
Outdoor metal furniture, railings, artifacts etc. take a beating from the elements. A few coats of polyurethane makes them more scratch resistant and weatherproof.
4. Easy to Apply and Maintain
Polyurethane can be brushed or sprayed on. Reapplication is easy when the sealant starts wearing off after a few years.
So in a nutshell, polyurethane takes your raw metal surfaces and gives them a finished, protected look and feel. But to enjoy these benefits, proper prep and application is crucial.
Preparing the Metal Surface
Applying polyurethane to an unprepared surface can lead to poor adhesion and premature peeling or chipping. Proper prep ensures that the poly bonds well to the metal. Here are the key steps:
Ensure the metal surface is free of grease, rust, mill scale, dirt, dust or grime. Start by washing with a mild detergent and water. For more ingrained grime, use a degreasing agent. Rinse and let dry fully.
Sand the Surface
This step is vital for polyurethane to adhere well. Use a medium to coarse sandpaper (80-150 grit) to scuff the surface. This helps the sealant grip better.
Be sure to sand in the direction of the grain and evenly across the surface. Sand curved or intricate pieces by hand.
Apply a Primer or Etching Solution
Bare metal and polyurethane don’t bond naturally. An intervening layer of primer provides the perfect gripping surface.
On steel, a self-etching primer formulated for metals works very well. Rustoleum and Krylon have good options.
For non-ferrous metals like aluminum, brass, copper etc., use a phosphoric acid-based etching solution. Let it sit for a few minutes before washing off and drying.
The metal is now ready for polyurethane application!
Applying Polyurethane to Metal Step-By-Step
Follow these steps closely to get a smooth professional-looking finish:
1. Prepare the Polyurethane
Stir the polyurethane container thoroughly before use. This ensures even distribution of solids and uniform brushability.
Warm the can slightly to improve flow and leveling if working in cold conditions.
2. Apply First Coat
Use a good quality synthetic bristle brush to apply the first coat. Move the brush slowly in long even strokes and spread the polyurethane as thinly as possible.
Alternately, use a paint sprayer on larger surface areas. Hold it 6-10 inches from the surface and apply thin passes to avoid drips and sags.
Work in sections to prevent the poly from drying before you finish spreading it. Maintain a wet edge.
3. Let the First Coat Dry
Allow the first coat to dry completely. This usually takes 2-3 hours. Proper drying eliminates brush marks and improves intercoat adhesion.
Drying time varies based on temperature, humidity, ventilation etc. Follow the manufacturer’s recommendations.
4. Sand and Wipe Down
Once dry, sand the first coat lightly with 220 grit sandpaper. This smoothens out any drips, dust or other imperfections.
Wipe off the dust with a tack cloth. This prepares a clean surface for the next coat.
5. Apply Second Coat
Applying the second coat just like the first using a fresh clean brush. Work efficiently to maintain a wet edge and prevent lap marks.
For spray application, apply thin passes until you achieve uniform coverage. Take care to overlap strokes.
6. Let the Second Coat Dry and Cure
Allow the second coat to dry for 2-3 hours like before. The curing process can take up to 2 weeks, so avoid heavy use during this time.
The poly will become clearer and harder as it cures. Humidity and temperature impact cure times.
7. Apply Additional Coats (if needed)
For surfaces that need extra protection, apply additional thin coats following steps 4-6.
Most metal surfaces need 2-3 coats for good coverage and protection. Let each coat dry 24 hours before recoating.
And your metal is now sealed and ready to use! Proper prep and application are key for long lasting results.
Does Polyurethane Prevent Rust on Metal?
Definitely! The polyurethane coating forms an impermeable seal that prevents moisture from reaching the metal. This effectively stops the metal from rusting.
However, here are some caveats to keep in mind:
- Polyurethane is water-resistant, not fully waterproof. With time, moisture can penetrate through imperfections.
- Any scratches or nicks in the finish expose raw metal and can lead to rust.
- Outdoor metal surfaces require more frequent reapplication as the sun wears down the protective layer faster.
So while polyurethane is very effective against rust, periodic maintenance and touch ups are needed, especially on metals constantly exposed to the elements.
Using Polyurethane on Aluminum and Painted Metal
Polyurethane works great on steel, iron, galvanized metal etc. But special considerations apply when coating aluminum or painted metal:
- Clean and sand as usual, then apply an etching primer formulated for aluminum.
- Lightly sand between coats for better adhesion.
- Limit to 2-3 thin coats to prevent cracking from builds up.
Painted Metal Surfaces
- Sand and degloss the painted metal before applying polyurethane.
- Adhesion promoters can be used for a better bond.
- Limit coats to 2-3 to avoid pulling up the existing paint.
So in a nutshell, poly can be used to seal both aluminum and painted metals with proper prep work.
To Sum It Up…
- Polyurethane makes an excellent protective finish for metal surfaces, preventing rust and adding gloss.
- Proper cleaning, sanding and priming prep work is vital for polyurethane to bond well to metals.
- Apply thin coats using a brush or paint sprayer, sanding lightly between coats.
- It resists moisture but requires periodic reapplication for continued protection, especially outdoors.
- On aluminum and painted metals, take extra steps to ensure proper adhesion.
When applied correctly, polyurethane is a easy and cost-effective way to give your metal surfaces a clear, polished and water-resistant finish. Just be sure to invest the prep time needed.
We hope this detailed guide gives you confidence to take on metal sealing projects with professional results. Now, look at some frequently asked questions about using polyurethane on metal.
FAQs About Using Polyurethane on Metal
Is polyurethane the best sealant for all metal types?
Polyurethane works well on most common metals – steel, iron, tin, copper etc. But it may not adhere as effectively to non-ferrous metals like aluminum. For aluminum, acrylic and epoxy-based sealers are better options.
Does weather affect polyurethane on metal?
Yes, outdoor conditions impact the durability and lifespan of the polyurethane coating. Moisture, sun exposure and dramatic temperature swings slowly degrade the protective finish. Outdoor metal surfaces need reapplication every 1-2 years for continued protection.
How many coats of polyurethane are needed on metal?
2-3 thin coats are recommended for most indoor metal items like appliances, railings, light fixtures etc. Outdoor surfaces may need up to 4 coats and more frequent reapplication every 1-2 years.
Does polyurethane yellow or discolor over time?
Polyurethane has good color stability compared to oils and varnish. However, it can develop a slightly yellowish patina over time, especially with UV exposure outdoors. This is more pronounced on light/white polyurethane.
Can you apply polyurethane over rust?
Never apply polyurethane over rust or it will peel off quickly. Always remove rust fully via sanding before priming and coating the surface.
What sheen is best for poly on metal?
On metal, semigloss and high gloss polyurethanes work best. The slight sheen enhances the natural look of the metal. Satin and matte finishes also work but subdue metallic luster.
Does temperature affect polyurethane application?
Yes, apply polyurethane only when temperatures are between 60-90°F. Colder temperatures can inhibit proper flow and drying. Hot temperatures make the poly dry too fast.
Can you use a foam roller to apply polyurethane?
Foam rollers are not recommended as they can create bubbles in the finish. Using a high-quality natural bristle brush for the smoothest professional finish is best.
Can you use polyurethane as car paint?
Polyurethane lacks the flex, adhesion and color pigments needed for automotive paints. It can be used as a topcoat sealer over properly prepared and painted metal car surfaces.
What are some alternatives to polyurethane for metal?
Common alternatives include lacquer, shellac, varnish, powder coating, acrylic enamels and epoxy paints. Each has pros and cons to weigh when choosing a metal sealant.
We hope these questions help clarify any aspects of using polyurethane on metal surfaces. Let’s now look at some parting thoughts.
If used properly, polyurethane can create a protective, polished finish on metal surfaces. The keys are thorough prep work, following correct application techniques and maintaining the finish.
On outdoor metal, more frequent recoatings may be needed. Light colors and gloss finishes work best. Proper temperature and humidity are imperative for good results.
With a bit of care and patience, polyurethane is an easy, affordable way to give indoor or outdoor metal surfaces a refreshed, durable look and protection against the elements.
We hope this detailed guide gives you the confidence to take on any metal sealing project with professional-quality results. Share your polyurethane tips and tricks in the comments below!