A method of forming a sealed cavity around an end of a fastener protruding from a surface of a structure, the method comprising: fitting a spark prevention cap over the end of the fastener; the spark prevention cap comprising a cap body with an annular base terminating at a base rim which surrounds an opening into a central cavity for receiving the end of the fastener; and the method further comprising: disposing a sealing material distribution tool for injecting sealing material around an external side of the spark prevention cap; and injecting sealing material through the sealing material distribution tool around the juncture of the structure and the cap, characterised in that the sealing material distribution tool comprises an annular tube, an annular passage in the annular tube, and a sealing material outlet from the annular passage with an annular configuration configured to distribute sealing material circumferentially about the spark prevention cap, wherein the sealing material distribution tool comprises a sealing material inlet to the annular passage, wherein the sealing material flows circumferentially from the inlet along the annular passage, and wherein the sealing material distribution tool is removed following application of the uncured sealing material. fitting a spark prevention cap over the end of the fastener; the spark prevention cap comprising a cap body with an annular base terminating at a base rim which surrounds an opening into a central cavity for receiving the end of the fastener; and the method further comprising: disposing a sealing material distribution tool for injecting sealing material around an external side of the spark prevention cap; and injecting sealing material through the sealing material distribution tool around the juncture of the structure and the cap, characterised in that the sealing material distribution tool comprises an annular tube, an annular passage in the annular tube, and a sealing material outlet from the annular passage with an annular configuration configured to distribute sealing material circumferentially about the spark prevention cap, wherein the sealing material distribution tool comprises a sealing material inlet to the annular passage, wherein the sealing material flows circumferentially from the inlet along the annular passage, and wherein the sealing material distribution tool is removed following application of the uncured sealing material. disposing a sealing material distribution tool for injecting sealing material around an external side of the spark prevention cap; and injecting sealing material through the sealing material distribution tool around the juncture of the structure and the cap, characterised in that the sealing material distribution tool comprises an annular tube, an annular passage in the annular tube, and a sealing material outlet from the annular passage with an annular configuration configured to distribute sealing material circumferentially about the spark prevention cap, wherein the sealing material distribution tool comprises a sealing material inlet to the annular passage, wherein the sealing material flows circumferentially from the inlet along the annular passage, and wherein the sealing material distribution tool is removed following application of the uncured sealing material.