In this article I’m going to explain how to polish aluminum bicycle parts. And in this case we’re going to polish a Sakae MTS-120 stem from 1986. As is the bike, the stem is in pretty bad shape with deep scratches and nicks all over.
If you polish aluminum bicycle parts you have to take the necessary steps to achieve the ultimate result of a mirror-finish. Obviously a good cleaning is in order. After which you sand the aluminum in a number of steps. Each step involves an increase in the number that represents the sandpaper grit. The final step involves using a machine polisher and both a cutting- and polishing compound.
Let’s get started.
Video Tutorial
Step 1. Cleaning the bicycle parts
This old stem is tightened with the help of a metal wedge-shape nut. By tightening the bolt that goes through the stem it moves itself against the inside of the head tube. It’s no exception that this wedge get’s seized within the head tube because of corrosion.
By loosening the bolt and tapping it you can easily loosen the wedge and get the stem out. But the obvious next step is to remove the rust.
After I’ve removed the rust I proceed to give the entire stem a general cleaning. This is necessary because you can’t sand and polish dirty parts.
Step 2. Sanding the aluminum bicycle parts
It’s not a bike restoration if the bike hasn’t been through a lot. And this bike certainly has. The stem contained deep scratches. Before you start polishing the aluminum needs to be sanded down to a dull finish. And of course you want to get the scratches out.
removing scratches wet sanding
I start off using an 80 grit sandpaper to remove all of the damage. If the damage is too extensive I stop at a certain moment to not compromise the shape of the part.
After the 80 grit I move up using 150 and 240 grit. These latter grits are used to smooth out all of the scratches left from the previous sanding step.
The final steps involve wet sanding with 400, 600, and 800 grit sandpaper. The machine polisher can pick it up from there.
Step 3: Machine polishing using cutting- and polishing compounds
On YouTube I’ve seen people go up to 3000 grit sandpaper before polishing. That’s only necessary if your machine polishing step does not involve a cutting compound. Because the cutting compound actually doubles down as a sanding step.
Everybody has their preferences when it comes to polishing. But for me getting to the machine polishing part sooner feels like saving a lot of trouble.
The machine polisher I use has two wheels on either side. A yellow one for cutting and a white one for polishing. I have four different compounds, 2 for each wheel, ranging from coarse to high-gloss.
The polisher makes around 3000 rotations per minute, or 50 per second. That means that the metal your holding can get hot to the point where it’s impossible to keep it in your hands without protecting yourself with a towel.
Final thoughts on how to polish aluminum bicycle parts
Polishing, although taking a tremendous amount of time, is one of the most satisfying processes of any bike restoration. And I can highly recommend purchasing a machine polisher to aid you in this process.
This bicycle stem is not the only part that needed polishing, and with this step taking up a couple hours alone, you might be tempted to skip on correctly sanding the part. I was. But you run the risk of needing to redo the sanding step because polishing with the cutting compound isn’t effective enough in taking out the scratches left by the sanding step.
I learned this the hard way and it effectively adds another hour to the entire process.
And that’s how you polish aluminum bicycle parts!
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