A sealing unit for sealing a gap between a pair of surfaces includes a nozzle for injecting sealant into the gap, rollers, an endless track round the rollers, and a curing device. The sealing unit is moved along the length of the gap. A sealant is injected into the gap from the nozzle and the track is rotated around the rollers as they are moved along the length of the gap. The track is pressed onto first and second surfaces so the sealant in the gap is constrained by the track to lie substantially flush with the first and second surfaces. The sealing unit is moved along the length of the gap at a rate such that the sealant becomes sufficiently cured when it is in contact with the track so that the track can be peeled away from the sealant as the track rotates round the second roller.