The rivet holes were not quite air tight.
Of course, it was built by Avro.
As I explained on another thread:-
It is not generally known that Avro built a prototype flying boat to compete for the Specification R.2/33 (which gave rise to the Sunderland). It was built in great secrecy at Woodford, taken in bits to a secret hangar next to the Manchester Ship Canal, and assembled.
Came launch day. It slid down the slipway into the water, and just went on going down, leaving nothing but a few bubbles. The whole thing was hushed up,and Avro never built another flying boat.