A cap for forming a sealed cavity around an end of a fastener. The cap has an inner cap terminating at an inner cap rim which surrounds an opening into a central cavity and lies in an inner cap plane around a majority of a circumference of the inner cap. An outer cap is fitted over the inner cap, the outer cap comprising an outer cap body and a skirt extending from the outer cap body to a skirt rim which lies in or slightly below the inner cap plane around a majority of a circumference of the skirt. An annular pocket is provided between the skirt and inner cap, and a flow space is provided between the outer cap body and the inner cap. The flow space is in fluid communication with the annular pocket so that it can receive a flow of sealing material and feed it into the annular pocket. The outer cap body is formed from a first material with a first elastic modulus, and the skirt is formed from a second material with a second elastic modulus which is lower than the first elastic modulus. The skirt flexes as the flow of sealing material is fed into the annular pocket so that the skirt rim lifts up to form a gap between the skirt rim and the structure which enables air to escape the annular pocket by flowing through the gap.