Common Metal Finishing Methods
There are many ways to finish metal.
You’ll want to ask yourself whether your finish is more for aesthetic or functional purposes before diving in. After you’ve made that decision, there are benefits and drawbacks to each that you may not have thought of. Hopefully, this brief overview will help you make that decision. This list is not exhaustive but is more common than some of the highly specific types of metal finishing.
Plating/Galvanizing
Plating is a process of using chemical baths to adhere to a thin layer of metal to coat the metal below. Some are more prevalent than others depending on your industry or product. For example: if you’ve ever shopped for home decor or fixtures you’ve certainly come across nickel-plated objects. In the industrial space, zinc plating or galvanizing is very common. There are many different ways to achieve a zinc coat on a product including but not limited to tumbling, molten coating, and cold coating.
Not only does galvanizing alter the appearance of steel, but it makes it more resistant to corrosion and friction. As with many finishing options, you’ll want to weld the product before galvanizing, if the product needs to be welded. This ensures an even galvanization. It can be welded after the fact, but the weld will not be resistant to corrosion, or the galvanization will have to be touched up (which is not as effective).
Galvanized coatings are usually cost-effective and warrantied for quite a long time. Some places say a lifetime while others say 75 years. The main disadvantages to galvanization are that the piece must be able to be submerged, and it must be transported to the treatment site.
Brushing/Cleaning
This is a mostly aesthetic finish and is only applied to metals that are corrosion-resistant by nature such as stainless steel, aluminum, and nickel. Brushing, cleaning, and use of abrasives is a pre-treatment procedure as well. This rids the surface of imperfections and creates a uniform grain surface texture for final treatment or coating. With naturally corrosion-resistant metals it can also be a final finish for aesthetic purposes.
Benefits include a smooth but textured appearance. This procedure can be done on-site and can be done before and after welding, it is also cost-effective. That being said, this type of finish is also more subject to normal wear and tear and may need to be touched up or refinished more often if subject to a high-use environment. It is not recommended for corrosive metals other than pre-treatment.
Coating (painting and powder coating)
Water and oil-based paints, as well as polymers, can produce a wide variety of colors and textures for a finished piece. Painting and powder coating also offer different levels of abrasion and UV resistance.
If you’d like a more detailed, read on the powder coating process you can find that here and why we usually recommend powder coating rather than painting. This difference has been written about quite a bit by myself and others, so I’ll spare the details.
Painting does offer a few benefits over powder coating: it can be applied on-site, it is widely available, and can be inexpensive. That being said, because it is plastic, powder coating is much more abrasion resistant and the finish is typically warrantied longer than a paint finish.
Weathering Steel
Typically, folks will call this type of steel COR-TEN. Fun fact, the COR-TEN registered trademark is still owned by U.S. Steel so COR-TEN is not available. When a supplier offers what they call COR-TEN they are usually referring to A606-4, A588, or A487 steel. These are all weathering steels. They all develop a layer of oxide film over the surface that slows down corrosion in the future. If you’ve ever seen a big steel structure or sculpture that has a dusting of surface rust that never seems to change, that’s probably weathering steel. You’ll typically find this type of steel in drier environments because it is not 100% rust-proof. If water is allowed to accumulate on the surface, those areas will corrode faster than the rest of the piece. What happens, is that the oxide layer continually regenerates on the surface when the material is subjected to the weather. Welding still needs to be considered because generally, the weld will corrode at a more “regular” rate if left uncoated.
The benefits of weathering steel are an easy, and relatively maintenance-free, corrosion-resistant finish that can be welded routinely.
That being said, weathering steel is more expensive than non-weathering steel. Depending on the project, though, the cost of weathered steel could be offset by the savings on finishing the product. The aesthetic of weathered steel is not for everyone, so you’ll want to take that into account too before deciding on that.
Sources:
1. https://www.ippmagazine.com/metal-finishes/
2. https://www.thomasnet.com/articles/custom-manufacturing-fabricating/types-metal-finishing/
3. https://www.corten.com/what-is-corten-steel.html
