the early tanks were of welded aluminium structure with the sealing material applied as a composite layer on the outside. a major problem with welded aluminium tanks is weld decay and metal fatigue in the seams due to vibration in service. in response to this, the company started to develop flexible fuel bladders in the early 1940s. one of the earliest examples of this was a composite flexible fuel bladder with self-sealing coatings for the supermarine spitfire mkix.