Aluminum does not rust, it oxidises — and oxidised aluminum goes bright, then grey, then chalky white. On a tanker or a trailer that reads as neglect even when the unit is mechanically perfect, and it happens fast on the Gulf Coast because heat, humidity and salt all accelerate it.
The reason soap does nothing is that the problem is not dirt sitting on the surface. It is aluminum oxide chemically bonded to the metal, and removing it means dissolving it.
The two jobs
Brightening
A controlled acid application dissolves the oxide layer and exposes clean metal. Dwell is short and monitored — acid left too long etches the surface it was meant to restore, which is the most common way this is done badly.
Neutralise and rinse
The acid is neutralised and thoroughly rinsed. Residual acid keeps working and blooms back through the finish within weeks.
Polishing
Mechanical polishing in progressive stages, where a mirror finish is wanted. This is labour rather than chemistry, and it is optional — a full trailer polish is a substantially bigger job than brightening the same trailer.
The mistake that cannot be undone
Anodised aluminum must never be acid brightened. Anodising is a protective oxide layer applied deliberately, and acid strips it — the damage is permanent and the surface will never look right again. Mill-finish, polished, coated and anodised aluminum all respond differently, so the finish is identified before anything is applied.
How long it lasts
Oxidation restarts as soon as bare aluminum meets air, so brightening is maintenance rather than a permanent fix. On a Gulf Coast unit, expect to revisit polished surfaces annually and brightened mill finish rather less often. Regular washing extends both intervals considerably, because it removes the road film that holds moisture and salt against the metal.