In the course of use the internal sufaces of cylinders may show signs of rust.  If the rusting is minor, then this will be removed by shot blasting the internal surfaces of the cylinder. Rust occurs due to the presence of moisture - this comes primarily from: 

• Storing cylinders empty and in particular with the valve open. 
• Filling cylinders with air that has not been filtered to remove moisture.  

Air fills provided by scuba diving centres produce filtered air that has moisture removed. 

NOTE: In the event of serious rust on the internal surfaces, the cylinder 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.