More frequent inspections are suggested. If deep pitting is observed, the cylinder should be removed from service. You must have the owners permission to remove their cylinder from service. The tank cannot be stamped for a lower pressure. It must be condemned.
Composite cylinders require testing every three years.
If the tank needs cleaning internally, it is essential that it be done before hydrotesting. If the tumble is done after, it could expose a hole, and it may leak through the wall when being filled. A tank with deep pits may pass a hydrostatic test, because the prime function of a hydro is to determine the elasticity of the metal. Tank boots should be removed before hydrotesting or false readings may occur due to salt deposits and trapped air bubbles. If severe rust is seen under the boot, have the tank boot area sand blasted before the hydro.