Metal Polishing 101

12 Apr 2022

Have you ever watched someone polish aluminum and zinc castings? A lot more goes into it than most people think! In this article, we will take a deep dive into metal polishing – from selecting a surface finish to tools and equipment.

Learn more by watching this recording of recent webinar with Danny Courter, BPI Polishing Lead with over 40 years of metal polishing experience.

Why polish aluminum and zinc castings?

Metal polishing creates a smooth, consistent surface. There are many reasons customers might want their castings polished.

  • Polishing prepares the part for powder coating, plating, anodizing, etc. Parts must be evenly polished before coating because coatings will highlight any imperfections or irregularities in the surface.
  • Polishing is used to make cosmetic, highly visible parts look aesthetic. It brings out a natural shine to the aluminum.
  • Polishing removes any burrs or pits that occur during the casting or machining process. Imperfections can weaken the casting or make parts unsafe to handle.

coated and uncoated parts

Different Surface Finishes

The level of surface finish is chosen by the customer, often determined by the part’s end application. Metal polishers use a combination of tools and series of operations to achieve the finish requested by the customer. Finishes we achieve at BPI include…

  • Grit number finishes (from 60 to 500) – The lower the grit, the rougher the surface finish. The higher the grit, the smoother and more reflective the surface finish.
  • Stainless Steel Shot Blasted – Small pellets of stainless steel shoot out around an aluminum casting to create an even, textured surface finish. The intensity of the texture is controlled by stainless steel shot frequency and cycle duration.
  • Satin – A softer finish with no directional orientation of the grain.
  • Mirror Buff – Mirror buffing creates a reflective surface. It is a time consuming and labor-intensive process that requires considerable skill. Mirror buffing is more commonly performed on zinc products, rather than aluminum.

It takes multiple steps to achieve these surface finishes. Polishers use multiple tools in a series of operations.

fire truck sirens

Sometimes, one part will have multiple surface finishes. For example, the A surface of a casting might be visible in assembly and require a 400-grit finish, while the B surface might be hidden and doesn’t require polishing at all. This is common in electrical housings or other casings.

Polishing vs Grinding

It is important to note that there is a difference between polishing and grinding. The goal of polishing is to remove as little of material possible, to hold the original contour of the casting. Grinding involves removing a large amount of material. Grinding might be performed to remove excess material or correct defects.

Polishing Tools and Equipment

Polishers use different tools to achieve different surface finishes. Perhaps two of the most common polishing tools are the polishing jacks and dual action sanders.

Polishing jacks are wheels with interchangeable grit belts. A dual action sander is a hand tool that is mainly used as a final operation. It has random and orbital settings to create an A1 finish. Often, these tools are used together. For example, the operator might start with the polishing jacks then complete the part with the DA sander.

batesville products aluminum polishing

Robotic Polishing

Metal polishing is traditionally a highly skilled art. The operator must use a variety of tools, apply even pressure, and balance perfection with efficiency to create an even surface finish. This type of finesse only comes with experience.

Today, advances in equipment and technology have turned this artform into a science. The polishing robot efficiently polishes parts by applying even pressure. It improves quality and consistency standards by ensuring that the same exact area is polished on each casting.

The robot essentially replaces polishing jack operations. After a part comes off the robot, we sometimes use the DA sander to finish the job.

polishing robot

I’m working on a new project. How do I pick a surface finish?

It can be daunting to decide the final look of your new product. Therefore, we often send samples of different finishes for customers to choose from.

When deciding what surface finish your new product should have, consider the following:

What is the end use of the product?

Does the look, texture, or durability of the product matter? For example, medical products often have different surface requirements than valves or hinges.

What are your industry standards?

Maybe you manufacture food equipment, and it is highly important that the surface is evenly textured.

Or maybe you produce safety equipment, where it is important that the product is easily accessible and identifiable. In this case, you might want to consider polishing and power coating your product neon yellow.

Is the product highly visible?

If the product is hidden in final assembly, maybe you can cut costs by removing polishing operations. On the flip side, it is important to make highly visible castings, such as these decorative architecture corners, aesthetically pleasing.

Will the “look” of the product add value?

If your company color is cherry red, polishing then powder coating your product will make it stand out from the competition!

Check out our casting gallery to see more surface finish examples!

It never hurts to check

Request a quote and a Batesville team member will be in touch to discuss your metalcasting goals and how our metalcasting experts can help make your next project a success.