How to mirror polish crank arms in 2 easy steps

Table of Contents

In this article you’re going to learn how to mirror polish crank arms. I’ll be hand sanding the crank arms using flint paper with various grits before moving into machine polishing to give it that mirror finish.

The crank arms came from my Cannondale Super V. That mountain bike received a new CODA Tarantula crankset because this one was so damaged. If I’d polished the crank arms earlier, I would have probably used the original Deore XT crankset.

Polishing any type of metal takes a lot of time if you want to do it right. Especially if the metal has signs of damage like nicks and dents besides surface scratches. The crankset I’ve polished has these. All in all it took me around 3 hours to achieve the result I was looking for.

Let’s get started.

Video Tutorial

Step 1. Polish crank arms using hand sanding

Crank arms are very prone to damage. Especially crank arms used on a mountain bike. Tree roots and rocks can damage the crank arms while pedaling as well as the occasional fall.

Besides these types of damage, crank arms will show signs of scratches caused by the heels of your feet when moving to the inside.

And of course, as the decades go by, shiny metal becomes dull with age.

That’s why the first step involves both removing damage as best as we can as well as smoothing out the surface and removing scratches.

Hand sanding with 60, 150 and 240 grit sandpaper

The first step is hand sanding with sandpaper and various grits. This first step takes the longest because it involves removing so much aluminum that it either diminishes them or removes them altogether.

It’s relatively easy to spot parts of the crank arms you haven’t done yet. Make sure that the entire surface of the crank arms look the same before moving onto the next grit level.

It’s boring and simple. But as it is with many things, preparation is key. Taking time for each step ensures you get the best result. There’s very little you can do to speed up this process and I wouldn’t advise using a machine. It’s always easy to mess things up that cannot be undone. And a machine sander will go so fast that irreparable damage is quickly achieved.

Wet sanding with 400, 600 and 800 grit sandpaper

The next step is to wet sand the crank arms. According to makezine.com:

The wet part of wet sanding refers to the use of water or some other liquid as lubrication to help carry away grit particles that are removed. Without the liquid, material can build up in the sandpaper and leave behind scratches that are larger than the particle size, ruining your finish.

Sandpaper for wet sanding isn’t made to dry sand so don’t try it. Make sure that you use enough water. Soaking the entire paper into a bowl of water is absolutely fine.

Wet sanding ensures that you get a smooth albeit dull surface, which is perfect before machine polishing.

I find that with the polishing machine I’m using, going up to an 800 grit wet sanding paper is smooth enough to not leave any marks when I’m finished with the entire process.

dull2 — A crank arm finished with a 1200 grit wet sanding paper. 800 is enough before machine polishing. You can still see a number of nicks at the back of the crank arm I decided not to remove.

dull — A crank arm finished with a 1200 grit wet sanding paper. 800 is enough before machine polishing. You can still see a number of nicks at the back of the crank arm I decided not to remove.

Step 2. Machine polish crank arms

I use a relatively cheap dedicated machine polishing tool and 4 types of cutting and polishing compounds.

The polishing machine has two discs made of cloth which are color coded (yellow and white for me). One is rougher than the other and together with the cutting compound serve as a very fine sanding material, comparable with sandpaper running into the thousands in terms of grit number.

removing compound — removing old compound

cutting compound — removing old compound

As with sanding, the first machine polishing step takes the longest. Using four compounds the first step takes roughly just as long as the three latter steps. It’s no wonder that my white cutting compound is by far the most used.

Make sure to buy the right compound. There are many variations. Your color code might not be the same as mine. My advice would be to buy the coarsest one at least and preferably an entire set, which is much cheaper.

Before polishing I remove the old compound by holding a 60 grit sandpaper against the polishing wheel.

Applying fresh compound is done by holding the compound stick against a rotating wheel. You’ll see the surface of the polishing wheel taking on the color of the specific paste.

When polishing aluminum this paste will immediately turn black. That’s supposed to happen. Don’t remove it but just keep on polishing. It would be a waste of your compound otherwise.

The end result

And that’s how you polish crank arms! If you like this article give it a thumbs up. If you have any questions or suggestions for new articles, let me know in the comments below. Cheers.

FAQ

Q: Can I repaint these aluminum bike parts?

A: Of course. An aluminium bike frame is painted. Though you need to set up the aluminium so your paint job will last. I would definitely not suggest to spray directly over fully polished aluminium because its adhesive nature is extremely low, causing the paint to flake very easily. Instead of explaining here what a proper paint job would entail I’d best refer you to a video from ETOE.