A cap for forming a sealed cavity around an end of a fastener protruding from a structure. The cap has: a cap body with an annular base terminating at a rim which surrounds an opening into a central cavity; an annular skirt providing an annular pocket between the skirt and the base; and an inlet hole in the skirt. The inlet hole is in fluid communication with the pocket and arranged to interconnect with a sealing material injection device to provide a flow of sealing material through the skirt via the inlet hole into the pocket. An outlet is also provided in the skirt. The outlet is in fluid communication with the pocket and arranged to enable air to escape the pocket through the skirt via the outlet hole as the sealing material flows from the inlet hole into the pocket.