What white worm is,
and why it spreads

White worm is corrosion happening underneath the lacquer on a diamond-cut wheel. It starts at an edge, spreads inwards as a pale, branching bloom, and it is the single most common reason a set of machined wheels stops looking right.

What is actually happening

A diamond-cut wheel is not painted silver. The face is machined on a lathe so the bare aluminium is exposed, and then a clear lacquer is laid over the top to protect it. That is why it has a fine turned grain and a brightness you cannot get from a spray gun.

Bare aluminium oxidises the moment it meets air and moisture. The lacquer is the only thing standing between the two. Once that seal is broken anywhere — a stone chip, a kerb scuff, or a microscopic gap at the rim edge from the day it was made — moisture gets underneath and starts corroding the metal from below.

The corrosion product takes up more room than the metal it replaced, so it pushes the lacquer up off the surface. That lifting is what you can see: a cloudy, worm-like white line following the path of the corrosion. Hence the name.

Why it always gets worse

Because each stage makes the next one easier. Corrosion lifts the lacquer, the lifted edge lets in more moisture, more moisture corrodes more metal, and the lifted area grows. There is no point at which it stabilises on its own.

Speed depends mostly on how much salt the wheel meets. On the coast — and Portsmouth counts — a wheel can go from a small mark at the rim edge to a third of the face in a couple of winters. Inland it is slower, but the direction is the same.

Two things accelerate it noticeably:

• Acid-based wheel cleaners. They attack the lacquer itself, which creates new entry points faster than the road ever would.
• Pressure washing at close range. A lance held inches from a lifted edge will peel it further in seconds.

What will not fix it

Worth saying plainly, because people spend money on all of these first.

1. Polishing. The corrosion is under a layer of lacquer. Polishing removes lacquer, which if anything opens it up further.
2. Touch-up lacquer. Sealing over corrosion traps moisture against the metal. It looks better for a month and comes back worse.
3. Painting the wheel silver. It will cover the bloom, and it will not stop it. It also loses the machined finish permanently, which is usually the reason you liked the wheels.

None of that is a reason to panic. It is a reason not to spend forty pounds at a motor factors before getting it looked at.

What actually does fix it

The lacquer has to come off, the corroded metal has to be machined away, and the face has to be re-cut and re-sealed. That is what diamond cutting is, and it is the only repair that addresses the cause rather than the symptom.

The catch is that cutting removes metal, and a wheel only has so much to give. Most will take three or four cuts across their life; some manufacturers allow fewer. We measure the remaining thickness before quoting, and if a wheel is at its limit we will say so rather than take one more skim and hand you back something structurally marginal.

Where a wheel cannot be cut again, the honest options are a painted or powder-coated finish — which looks different but lasts well — or a replacement. We will tell you which we would do.

How to slow it down

If your wheels are fine now, this is the part worth acting on.

• Use a pH-neutral wheel cleaner. It is the single biggest thing you control.
• Rinse the wheels after a winter drive, particularly if you are near the sea.
• Keep the pressure washer a sensible distance from the rim edge.
• Get chips and kerb scuffs sealed early rather than waiting until the whole set needs doing.
• Consider a ceramic sealant after any refinishing. On the coast it earns its money.

If you are not sure whether what you are looking at is white worm or just dirt trapped under the lacquer, send a photograph. Daylight, straight on to the rim. It takes ten seconds and it costs nothing to ask.