Aluminium oxidisation can be removed with cotton wool dabbed with a little vinegar. Thereafter, fairly regular (depending upon usage) cleaning with soapy water and thorough drying should keep your ROK in excellent condition. Full cleaning instructions can be found under the FAQ section of our website.
If the vinegar does not work, I would suggest trying a metal cleaner to remove the oxidisation with sand paper (Wet & Dry) and then a light coating of petroleum jelly prior to re-assembling the cylinder. This will ease any future removal.