Polishing metal is somewhat akin to sanding wood: You start coarse and end fine. You may even begin by cleaning the part you're working on with sandpaper to remove nicks and scratches before you turn on a power tool. But once the rough work is out of the way, you'll shift to metal-polishing compounds, which are waxy materials loaded with grit. When you apply it to the correct polishing wheel, the compound sticks, and the magic of bringing a real shine to metal can begin.
With the proper tools and polishing compounds, you can make almost any metal object (tools, engine parts, etc.) gleam as if it just came from the foundry.
The equipment you'll need to polish metal is pretty simple: a polishing machine and an assortment of wheels and abrasives. The machine is a task-specific tool with longer shafts than a bench grinder (which it resembles). Those long shafts create enough clearance that you can mount larger polishing wheels for increased surface contact, and they allow for more room to maneuver the item you're polishing (see photo below). If you already own a bench grinder, you can use it for polishing, but you'll need to remove any guards that would get in the way of the polishing wheels.
A dedicated polishing machine on a pedestal stand will deliver the best performance. This unit from Craftsman, complete with a stand, costs about $200.
A corded drill can also be used as an effective polishing machine, especially when you're working in tight quarters or on parts that you can't remove and bring to a bench (see photo below).
When you can't remove a part to bring it to the bench, you can use a corded drill to achieve similar polishing results.
Different polishing wheels perform different tasks. The most common wheels are made from cotton and come in a variety of sizes (see photo below). Sisal wheels are impregnated with fibers similar to stiff twine that produce a rougher cut and are ideal for beginning the polishing process. Light, minimally stitched wheels are softer and allow you to really press the part you're polishing into them — ideal if there are lots of small nooks or crevices in the workpiece.
Polishing wheels are available in a variety of sizes and types. Shown here (from top left going clockwise) is a tightly woven 6-in. cotton wheel, a looser 6-in. cotton wheel, an 8-in. very loose wheel for deep nooks and details and an 8-in. sisal wheel for more abrasion.
If you don't have a polisher, you can use a hand drill outfitted with a variety of shaped polishing bits such as these. These are ideal for working on parts that can't be removed and brought to the bench.
For polishing wheels to work, they need to be coated with an abrasive compound (see photo below). Generally speaking, compounds range in grit from dark to light, with black being the coarsest, then gray, brown and white, which are progressively finer. Blue and red bars of compound are designed to buff precious metals or metal plating to a brilliant shine and have no abrasive qualities, and green compounds are typically intended for stainless steel.
Polishing compounds are organized by color — darker colors such as black and gray are the most abrasive; lighter colors such as white and red are much softer and better suited for buffing or polishing precious metals.
You'll need to prep the piece to be polished by first removing any debris (which, when spinning at several thousand revolutions per minute [RPM], would have its own abrasive effect). Sand away larger scratches and imperfections because attempting to buff them out will only create large divots. Gradually work from coarse sandpaper to finer paper or steel wool before you move on to polishing compound.
After applying an abrasive compound, notice how the fibers in this sisal wheel stand out so that they can remove deep blemishes.
The idea when polishing is to let the abrasive compound do all of the work, and that requires speed. Professionals measure this effect through a term called surface feet per minute (SFPM). To determine the SFPM, you multiply the RPM of the polisher's motor by the wheel diameter and then divide by 3.82. The goal when polishing is to be between 4,000 and 7,000 SFPM. So for example, if we know that the buffer rotates at 3,450 RPM and we're using an 8-in. wheel, the equation would be (3,450 x 8) ÷ 3.82 = 7,225 SFPM, close enough to our 7,000-SFPM limit to be acceptable.
To remove stubborn corrosion or imperfections, work against the rotation of the wheel to create a cutting effect, as shown with this twist bit.
You'll apply polishing compound to the polishing wheel as it spins. Apply the compound lightly and frequently or the wheel will simply fling the substance all over you and your workspace.
Touch the workpiece to the polishing wheel (see "Working Smart" at the end of this article for tips on positioning the workpiece in relation to the wheel). Don't forcefully push the workpiece into the wheel (that's only necessary when you need to get into tight places). Slowing down the wheel's speed by force only slows down the polishing process (see photo below).
Work into the nooks and crevices to remove all corrosion and discoloration. Don't push too hard, as you'll only slow the polishing process.
As you progress from coarse compound to finer compounds, remember to change the wheel — use a different one for each type of compound. When working with coarse compounds to remove stubborn corrosion or imperfections, move the workpiece against the rotation of the wheel to create a cutting effect. But when you use finer compounds, work with the rotation of the wheel to create more of a buffing effect.
The piece you're working on will dictate how much preparation and polishing each stage will require. It takes more work to bring a shine to harder metals such as iron and steel than it does to polish aluminum or brass. But with the right tools and techniques, you can achieve brilliant results on any type of metal.
Polishing results can be dramatic: The shiny bit looked identical to the other before it was polished.
Whenever you're polishing metal, safety is paramount. Remove all jewelry, including your watch, so it can't get caught by a wheel. Always wear safety goggles and gloves — metal can get very hot during the polishing process. Wear a protective apron or coveralls, as the dust from the process can be messy and sticky, and a dust mask to keep you from inhaling any airborne particles. Pay attention to the position of your workpiece in relation to the polishing wheel. If the wheel is rotating downward, position the workpiece on the bottom half of the polishing wheel. Conversely, if the wheel is rotating upward, ensure that the part is on top. That way, if you lose your grip on a part, it will be projected away from you.