Polished aluminium is the cheapest dramatic change you can make to an old bike, and the easiest to ruin permanently. The process is simple and unforgiving in equal measure.
The grit ladder
You are not polishing. You are sanding progressively finer until the scratches are too small to see, and then buffing what is left. The polish at the end takes ten minutes. Everything before it takes three hours.
Start at 400 grit only if the casting is genuinely rough or corroded. On a typical side cover that has just gone dull, 800 is your starting point. From there: 1000, 1500, 2000. Wet, always, with a drop of washing-up liquid in the water to stop the paper clogging.
The one trick that matters
Change your sanding direction ninety degrees each time you change grit. Sand lengthways at 800, across at 1000, lengthways again at 1500.
This is not superstition. It is how you know when to stop. When every scratch running across the part has disappeared, the 1000 grit has done its job and you move on. Without the direction change you are guessing, and guessing is how people spend an hour at 2000 grit trying to remove 800 grit scratches that are still there.
Change direction ninety degrees between grits. It is not superstition, it is how you know when to stop.
Buffing
One compound is enough for most work. A brown or grey cutting bar on a stitched wheel, then the same wheel wiped clean, or better, a second loose wheel for finishing.
Light pressure, keep the part moving, and never present a sharp edge into the direction of wheel rotation. The wheel will grab it, throw the part across the workshop and put a gouge in it that means starting again at 400.
Where the swirls come from
A loaded buffing wheel. Compound builds up, hardens, and starts cutting like sandpaper instead of polishing.
Rake the wheel with a wheel rake or the corner of an old hacksaw blade every ten minutes of use. If you find yourself pressing harder to get results, stop and rake it. That is the moment swirls appear.
Keeping it
Bare polished aluminium dulls in weeks. Either accept that and re-buff twice a year, which takes twenty minutes once the hard work is done, or lacquer it and accept that lacquer will eventually yellow and have to be stripped.
We do not lacquer. Twenty minutes twice a year is less annoying than stripping lacquer off a finished part.
The grit ladder
| 400 grit | Only for rough castings or corrosion | 20 min |
|---|---|---|
| 800 grit | Normal starting point on dull alloy | 45 min |
| 1000 grit | Sand across the previous direction | 35 min |
| 1500 grit | Change direction again | 30 min |
| 2000 grit | Surface should already look near-polished | 25 min |
| Cutting compound | Stitched wheel, light pressure | 15 min |
| Finishing | Clean or loose wheel | 10 min |
| Total | Per side cover, first time | ~3 hours |
Common questions
How do you polish aluminium without swirl marks?
Swirls come from a loaded buffing wheel, not the wrong compound. Rake the wheel clean every ten minutes with a wheel rake or an old hacksaw blade. If you need to press harder to get a result, stop and rake it.
What grit should you start at when polishing alloy?
800 grit for normal dull aluminium. Only drop to 400 if the casting is genuinely rough or corroded, because every coarser grit you use is more work to remove later.
Why change sanding direction between grits?
So you can see when each stage is finished. When all the scratches running across the part have gone, the current grit has done its job. Without the direction change you are guessing.
Should you lacquer polished aluminium?
It is a trade. Lacquer keeps the shine but eventually yellows and has to be stripped, which means starting the whole process again. Re-buffing bare alloy twice a year takes about twenty minutes.
Where can I see more of this build?
The full photo set for every build is on Instagram at @ironandgasolineofficial, and questions get answered there.
Got a question about this build, or built one yourself? The conversation happens on Instagram — the full photo set for this bike is up there, and I answer everything in the comments.
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