Aluminium is not one surface. Bare cast aluminium can oxidise and stain, polished aluminium can be scratched by abrasive cleaners, and anodised aluminium can be clouded permanently by strong alkaline or acidic products. Use a pH-neutral cleaner, rinse promptly and test on a hidden area.
Identify the finish
Bare aluminium is dull silver and oxidises quickly in humid air. Polished aluminium is bright and reflective, usually with a protective wax or clear coat. Anodised aluminium has a coloured or clear oxide layer that is part of the surface, not a coating you can polish back.
Clear-coated aluminium looks like bare metal but behaves like paint: solvents and abrasives can damage the clear layer. If a part has a uniform sheen and a sharp edge where it meets a machined surface, it may be coated.
What damages aluminium
Strong alkaline cleaners etch bare aluminium and can turn it dark or chalky. Acidic wheel cleaners attack it quickly, especially when warm. Left to dry, either can leave a permanent stain.
Abrasive polishes remove the surface layer. On bare aluminium that can be corrected with more polishing; on anodised aluminium it removes the colour and leaves a patchy surface that cannot be restored without re-anodising.
Cleaning bare and polished aluminium
Start with a pH-neutral shampoo and a soft brush. Rinse before the cleaner dries. For oxidised bare aluminium, a dedicated aluminium cleaner or a mild metal polish can restore brightness, but work in shade and rinse thoroughly.
Polished aluminium benefits from a wax or sealant after cleaning. Without protection, it oxidises again quickly, especially near the coast. Use a non-abrasive sealant and buff lightly.
Anodised parts
Anodised surfaces need the mildest approach: pH-neutral shampoo, water and a soft cloth. Do not use metal polish, abrasive pads, strong degreasers or acidic wheel cleaners. If a stain will not lift, accept it or consult a specialist; aggressive cleaning will make it worse.
Coloured anodising can fade with UV over years. That is a change in the oxide layer, not dirt, and no cleaner will bring the colour back.
Fasteners, dissimilar metals and galvanic corrosion
Aluminium and steel in contact, with water between them, form a small battery. The aluminium becomes the sacrificial anode and corrodes first, often as a white powder around a steel bolt. This is galvanic corrosion, and it is why fasteners on an aluminium engine or frame deserve attention out of proportion to their size.
Clean the area, dry it and apply a suitable barrier: an anti-seize compound on the threads, a thin protective film on the joint face, or the manufacturer's specified treatment. Do not substitute copper-based anti-seize on aluminium without checking, because some formulations can worsen the reaction.
Inspect engine mounts, suspension linkages and bodywork brackets. A corroded mount can hide behind a fairing until a bolt starts to loosen. If a fastener turns too easily, or feels gritty, stop and investigate rather than tightening it further.
Riders who live near the coast should treat this as routine, not as a restoration task. A few minutes with a brush, fresh water and a barrier compound after a coastal ride is cheaper than extracting a seized bolt from an aluminium casting.
Oxidation, corrosion and the coastal factor
Aluminium forms a thin oxide layer that protects it, but salt and moisture can drive deeper corrosion, especially where two dissimilar metals touch. Rinse coastal bikes with fresh water after rides and keep drain holes clear.
White powdery corrosion around fasteners is a warning. Clean it, dry it and apply a suitable protectant. If a fastener is corroded into aluminium, do not force it; heat and penetrating oil may help, but a workshop is the safer route.
Polished aluminium also benefits from a periodic check of the lacquer or wax layer. When water stops beading, the protection has gone and oxidation has started. Reapply a non-abrasive sealant before the surface dulls, and avoid polishing a coated part unless you intend to remove the coating entirely.
Remember that aluminium is soft. A dropped spanner, a grit-filled cloth or a screwdriver used as a lever leaves a mark that no cleaner will remove. Work on a padded surface when a part is off the bike, and keep fasteners in a tray rather than on the floor.
Checklist
- Identify bare, polished, anodised or clear-coated aluminium.
- Use pH-neutral shampoo and a soft brush.
- Never let a strong cleaner dry on aluminium.
- Keep metal polish off anodised and clear-coated parts.
- Rinse coastal bikes with fresh water after riding.
- Protect polished aluminium with a non-abrasive sealant.
Questions
Can I polish anodised aluminium?
No. Polishing removes the anodised layer and leaves a patch that cannot be restored without re-anodising. Clean it gently and protect it.
Why did my aluminium turn white after washing?
That is likely etching from an alkaline or acidic cleaner left to dry. Rinse promptly and use a pH-neutral product. Light etching may polish out on bare aluminium; anodised damage usually cannot.
Is WD-40 safe on aluminium?
It can displace water and leave a film, but it is not a cleaner and it attracts dust. Use it sparingly and keep it away from brakes and tyres.
