A change in linear welding conditions from those previously used, namely higher vibration frequency (150 Hz) and lower displacement (2 mm) during welding afforded a quicker rise in temperature of the weldline. This allowed a much shorter welding time (1.5 s). This resulted (a) in no fibres being expelled from the welded interface edge and (b) the thin exposed interface seen on the joint side not being dark, indicating almost no charring of the interfacial region. The maintenance of all the fibres in the interfacial composite ensured a stronger joint and a much improved water resistance due to the decrease and almost absence of the rather porous weldline char which would have facilitated water penetration. Such change in the set of parameters allowed resistance of the linear welded joints in water for more than 25 h.