Sheet metal fabrication is one of the most important processes in the current manufacturing industry. Over 60% of the parts in industries such as automotive, aerospace, and electronics require sheet metal fabrication. Cutting and bending is the process of giving the metal its desired form. However, the sheet metal finish is a very important factor when it comes to the functionality and the visual value of the part in the long run. Understanding the full metal fabrication process helps ensure proper material handling and surface finish selection.
A proper sheet metal finishing process reduces the chances of corrosion, enhances wear resistance, and meets the standards of the industry. For example, medical devices may require FDA-grade smooth surface finish while architectural cladding may require UV protection. On the other hand, if the wrong finish is chosen then it can lead to various problems. Components may wear out before their time, maintenance expenses may go up, and the brand image may be damaged.
To this end, experienced engineers select the right finish during the prototyping stage rather than during the production process. This guide outlines 12 key finishing processes. These methods assist you in selecting the right surface treatments for your design, environment, and part life cycle from the first design sketch on the CAD.
What Is Sheet Metal Finishing?
Sheet metal finishing is the last process in the fabrication process of a particular product. It applies some treatments to the raw metal parts to make them more durable, functional, and aesthetically appealing. These treatments are used after operations such as cutting, bending or welding among others. In general, finishing has three primary objectives:
Protection: Guards metal against corrosion from saltwater, chemicals, and other elements. It also does not wear or get damaged easily by ultraviolet radiation.
Purpose: Enhances certain characteristics of the material like electrical conductivity, lubrication, or heat resistance.
Aesthetics: Provides the required color, shine or feel on the surface of the parts as per the brand image.
Types of Sheet Metal Finishes
Black Oxide
Black oxide converts the surface of steel into magnetite iron oxide Fe₃O₄. This is done by placing the steel in a heated solution of sodium hydroxide and nitrates. The process forms a black, anti-corrosive layer that is almost as thin as the original layer, 0.5 to 1.5 μm. It is best suited for small items like gears, fasteners, and military hardware where size tolerance is crucial.
Besides, black oxide offers a sleek, matte surface that can improve the abrasion resistance of sheet metal parts. When mixed with oil or wax, it seals tiny pores on the surface and enhances the protection even more. However, this method is not suitable for stainless steel or non-ferrous metals and is only suitable for standard steel parts.
Electroplating
Electroplating refers to a technique that applies an electric current to deposit a layer of metallic thickness varying between 0.5 to 25 μm on metal surfaces that are conductive such as steel or copper. It is used for three primary uses: corrosion protection, wear, and aesthetics.
Zinc plating acts as a sacrificial anode. It corrodes before the underlying steel, making it suitable for automotive bolts and outdoor hardware. Nickel plating is used to increase the life of parts such as hydraulic pistons while chrome plating gives a shiny, mirror-like surface finish that is common in consumer products. For components that need to have electrical conductivity, gold plating is ideal, especially for microchip connectors.
However, electroplating has some limitations. It is ideal for simple shapes because complex shapes, such as deep grooves or cavities, have issues with current distribution and, therefore, receive an uneven coating. Complex geometries may require further polishing or masking. Note, the thickness of coat affects its longevity—and hence a thicker coating is more durable, but if you decide to go with a thicker coating you will have to spend more money. In functional applications, the coatings should meet standard requirements such as ISO 2081 for zinc or AMS 2423 for nickel.
Anodizing
Aluminum anodizing is a popular finishing technique for aluminum products. This process involves placing the aluminum in an acidic electrolyte and passing an electric current through it. This creates a passive oxide layer that is between 10 and 100 μm in thickness. The finish that is obtained has excellent wear resistance and can also be dyed in various colors. Anodized colors are not painted on the surface but rather they penetrate the oxide layer and therefore do not peel off. The oxide is later sealed using boiling water to lock in the dye.
Anodized layer is much harder than untreated aluminum, making it highly durable. Sulfuric acid is used in type II anodizing, which is most often used for architectural purposes. For aerospace components, the thicker and harder Type III (hard coat anodizing) is used. Also, anodizing improves electrical insulation, and thus, it is widely used in electronic applications such as heatsinks.
Painting
Painting is a surface finishing process that involves the application of liquid coatings on sheet metals. It offers corrosion protection and has a high degree of appearance versatility. Some of the common types of paint are epoxy, acrylic and polyurethane. Epoxy paint is resistant to chemicals and therefore suitable for use in industrial machines. Acrylic paint dries quickly and is ideal for indoor furniture. Polyurethane paint is resistant to UV light and therefore ideal for use in outdoor advertising.
The metal surface is first prepared by cleaning it. Sometimes, we use a phosphate conversion coating to enhance the adhesion of paint. To apply the paint, spray guns are used to paint the parts uniformly and then the painted parts are baked in an oven at 120-150°C. This makes the finish more rigid and thus more resistant to wear and tear.
Painting is cost-effective and can be done in any color, including RAL or Pantone-matched colors. However, painted surfaces are not as durable as powder coatings since they can chip or fade with time. However, painting is a good option for such products as consumer appliances, architectural panels, or prototypes that need quick and customizable designs.
Powder Coating
Powder coating is a technique of applying dry powder material to the surface of the metal through electrostatic charges. The coated surface is then baked to make it hard, smooth, and strong to ensure that it will not wear out easily. Unlike traditional liquid paint, powder coating emits no VOCs (volatile organic compounds). It also produces a heavier coating that is between 60 and 120 micrometers thick, which is less prone to chipping than paint.
Also, powder coating is good at concealing small surface defects that may be present on the surface of the material. It also provides hundreds of finishes, including glossy metallics or textured wrinkle finishes. However, its thickness can be a problem for some parts such as precision gears or parts that are closely fitted.
Galvanizing
Galvanizing is a process of applying a thin layer of zinc on steel to prevent rusting of the metal. The most common one is hot-dip galvanizing. This is done by dipping the steel in molten zinc at a temperature of 450°C to form a thick and strong layer. It is ideal for structures that are exposed to the environment such as streetlights or guard rails. In the case of small parts such as bolts, electro-galvanizing uses a thinner layer of zinc through the process of electroplating and hence has a smoother surface.
Galvanizing is most useful in areas where the climate is unfavourable and where there is a lot of exposure to salty air or other corrosive agents. The zinc coating has a self-healing ability that covers small scratches that may be present on the surface hence little maintenance is required. However, it has a dull gray color and a relatively coarse structure, which may not be suitable for decorative purposes.
Phosphating
Phosphating is a surface finishing process that creates a thin protective layer on steel parts. It is primarily used as a pretreatment step to improve paint or powder coating adhesion. Common applications include car chassis components and machinery that require both corrosion resistance and a decorative finish.
There are two main types of phosphating. Zinc phosphate is used for parts that require lubrication, such as engine mounts. Iron phosphate, on the other hand, is a cost-effective option for indoor applications. Phosphating has minimal impact on tight tolerances, making it suitable for bolts, threaded rods, and other precision parts.
However, phosphating is only effective on carbon steel. It must be paired with a topcoat for outdoor use to provide long-term corrosion protection. This method works well for parts that experience moderate wear or need their coatings to last longer.
Passivation
Passivation improves the inherent corrosion protection of stainless steel by removing such things as free iron on the surface of the material. It is suitable for 300 series (for example, 304, 316) and 400 series stainless steels that are used in areas where they are exposed to moisture or chemicals. This step restores the natural oxide layer on the metal surface which is important for use in medical or food industry. But it cannot remove rust or provide protection to carbon steel in any way.
Buff Polishing
Buff polishing uses rotating cloth wheels covered with abrasive compounds to create shiny or satin-like finishes on sheet metal surfaces. This method is ideal for removing light scratches and oxidation. It works particularly well on stainless steel and aluminum parts that require a polished and appealing look.
For small batches, manual polishing allows customization of the finish. However, it is very labor-intensive and becomes expensive for large-scale production. Automated polishing systems are more efficient for flat or gently curved panels. However, they are less effective for parts with intricate engravings or detailed patterns. Additionally, for outdoor applications, it’s best to pair buffing with a protective coating, like clear lacquer, to prevent tarnishing. Avoid buff polishing on parts with deep recesses, as polishing residue can get trapped and be difficult to remove.
Sandblasting
Sandblasting is a sheet metal finishing process that involves the use of abrasive materials like glass beads, aluminum oxide or steel grit at high velocity to clean or texture metal surfaces. It effectively removes rust, mill scale and old coatings. It also gives a uniform matte finish and is ideal for surface preparation for painting or achieving a brushed appearance.
The grit size determines its effectiveness. Coarser grits (80-120) are suitable for the removal of thicker contaminants or for rough surface finishing. Finer media (220+ mesh) are used for finishing applications like polishing the stainless steel enclosures. This technique improves paint adhesion and hides minor surface imperfections by increasing surface area. However, high-intensity blasting may reduce the thickness of the metal, particularly for delicate parts.
Electrolysis Polishing
Electropolishing is an advanced process that smooths metal surfaces by removing surface irregularities using an electric current. This method dissolves the peaks of the metal, resulting in a smooth, hygienic surface. It also strengthens the natural oxide layer, which improves corrosion resistance. As a result, electropolishing is commonly used for stainless steel parts exposed to harsh chemicals or sterilization processes.
Though electropolishing is more expensive than other finishing techniques, it delivers consistent results, even on intricate shapes such as tubing interiors. However, it works less effectively on certain metals like aluminum. This method is ideal for applications requiring FDA-compliant surfaces, such as in pharmaceutical or semiconductor industries.
Brushing
Brushing produces a consistent linear or radial grain pattern on metal surfaces. This method uses abrasive belts or nylon wheels to create a matte finish. Brushed finishes are excellent at hiding minor scratches and fingerprints, which is why they’re commonly used for automotive interior parts like door sills or gearshift panels. These applications often prioritize scratch resistance and easy cleaning.
Grain direction can be tailored to specific design needs, such as horizontal, vertical, or circular patterns. Because the grain pattern can impact the product’s overall look, it’s important to decide on the brushing design early in the development process. This ensures the final product meets aesthetic expectations.
How to Choose the Right Sheet Metal Finish for Your Project
Matching Finishes to Materials
Not all surface finishes work with every metal. For instance, anodizing suits aluminum, while galvanizing is better for steel. Always check compatibility to avoid adhesion issues or premature wear. Below is a general guide for common sheet metal materials:
| Finish | Carbon Steel | Stainless Steel | Aluminum | Copper Alloys | Galvanized Steel |
|---|---|---|---|---|---|
| Black Oxide | ✅ | ❌ | ❌ | ❌ | ❌ |
| Electroplating | ✅ | ❌ | ❌ | ✅ | ❌ |
| Anodizing | ❌ | ❌ | ✅ | ❌ | ❌ |
| Painting | ✅ | ✅ | ✅ | ✅ | ✅ |
| Powder Coating | ✅ | ✅ | ✅ | ✅ | ✅ |
| Galvanizing | ✅ | ❌ | ❌ | ❌ | ✅ |
| Phosphating | ✅ | ❌ | ❌ | ❌ | ❌ |
| Passivation | ❌ | ✅ | ❌ | ❌ | ❌ |
| Polishing | ✅ | ✅ | ✅ | ✅ | ✅ |
| Sandblasting | ✅ | ✅ | ✅ | ✅ | ✅ |
| Electropolishing | ❌ | ✅ | ✅ | ❌ | ❌ |
| Brushing | ✅ | ✅ | ✅ | ✅ | ✅ |
Note: This table provides a baseline—real-world applications often require adjustments. For complex projects, consult a sheet metal fabrication manufacturer to validate your finish selection.
Environmental Exposure
It is essential to consider whether your part will be exposed to saltwater, UV rays, or chemicals. When choosing the type of finish, it is important to ensure that the treatment is appropriate for the operating environment.
Humid/Coastal: Choose Galvanizing (withstands 500+ salt spray hours) or Electroplating
Chemical Exposure: Opt for Passivation (acid resistance) or Powder Coating (solvent protection)
High Temperatures: Anodizing does not degrade up to 500°C.
Outdoor UV Exposure: PVDF Coatings prevent fading (20+ years of color retention)
Abrasive Conditions: Sandblasted surfaces do not get scratched easily
Food/Medical: Electropolishing removes micro-pores for cleaning
Mechanical Requirements
Wear resistance is very important in sheet metal products, especially in applications where friction and heavy loads are involved. Lack of wear protection results in the deterioration of the part and its failure or reduced performance, which affects its durability and safety. When choosing the right surface treatment, one must consider the type of friction that is sliding, rolling, or impact, and the load and frequency of friction that the part will experience. Components that experience a lot of wear require coatings such as hard anodizing or electroplating. For moving components, the main requirements are low friction and wear.
Aesthetic Preferences
Aesthetic preferences also function as one of the most important factors when determining the type of surface treatment. Brushing gives a more refined look to the products while powder coating gives products a more vibrant color. Anodizing offers a tough and colorful surface that is ideal for products where aesthetics are important. For industrial or hidden parts, it is possible to use galvanizing or raw metal since such parts do not have to be aesthetically appealing. Both packaging and color should, therefore, be selected depending on a product’s type and visual fit.
Cost Consideration
Electroplating and custom finishes such as electrolysis polishing can be costly. For cost-sensitive applications, it is possible to simply paint or use standard finish. It is advisable to consult with your fabricator as soon as possible because specialists can advise on the best finish combinations (for example, galvanizing and passivation) that will be cost-effective and durable. A DFM (Design for Manufacturing) check can be made to avoid over-engineering.
Conclusion
Choosing the right sheet metal finish depends on material, environment, function, and budget. Whether you need corrosion resistance for stainless steel parts or a vibrant powder-coated finish for consumer products, each option offers unique benefits. Still unsure? Partner with a fabrication shop that understands the nuances of metal finishing—your project deserves nothing less.
TZR: Your Primary Sheet Metal Fabrication Company for all Finishing Needs
TZR is a leading sheet metal fabrication company that provides design, prototyping, and manufacturing of sheet metal products for automotive, medical devices, 3D printers, and renewable energy industries. Our team has been working for more than 30 years and we are very good at dealing with materials such as stainless steel, aluminum and copper. We also provide a variety of sheet metal finishes including powder coating, anodizing and electroplating and all are done internally. This commitment to accuracy enables us to offer up to 21 finishing processes in order to satisfy the needs of the clients. TZR has been serving clients in more than 30 countries and offers durability, functionality, and beauty.
