Salt stains on aluminum dump truck boxes

[jakeman]

Does anyone know of a way to get the salt stains off aluminum dump truck boxes without using acid. I have been cleaning alot of gravel trucks over the winter and now that the nicer weather is coming they all want those ugly salt stains removed but don't want acid used. I tried brushing them with soap but as soon as they dried the stains were back. Any tips would be appreciated. Thanks