If the external surfaces of a steel scuba cylinder have any areas of significant rust, then they must be cleaned, galvanised (a process which involves the external surface of the cylinder being electroplated with zinc which prevents the steel from rusting) and repainted to protect the galvanised surface from damage. 

NOTE: Please be aware that cylinders with severe external corrosion will not be returned and will be destroyed - this is an HSE requirement because they are no longer safe to be filled due to the risk of catastrophic failure.