The depth of moisture cure (z) has been measured for lap joints in aluminium with a conventional sealant based on polydimethylsiloxane and one based on polytrifluoropropylmethylsiloxane. The cured sealant acts as a barrier to the permeation of water; any water which passes through this barrier causes further cure which thickens the barrier. The depth of cure is given by z=(2PVpt)(1/2), where P is the permeability coefficient of water in cured sealant, V is the volume of sealant which reacts with 1 mole of water, p is the vapour pressure of water and t is time.