Complex sentence: A vast majority all of the B-29s were stored by a new process of cocooning, however, this process trapped heat and moisture resulting in numerous airframes being damaged by this process (primarily the avionics and instruments).
Split into simpler sentences: A vast majority all of the B-29s were stored by a new process of cocooning. However, this process trapped heat and moisture resulting in numerous airframes being damaged by this process (primarily the avionics and instruments).