Abstract: In the progressive forming and laying of an undersea pipeline the welded joints between the ends of pipes pre-coated with a corrosion protection coating and an outer concrete weighting coating are protected by a mastic coating applied by clamping a mould around the pipeline joint to form a cavity between the joint and the mould, filling the cavity with mastic by forcing into it under pressure a charge of mastic at a temperature of not more than 70.degree. C., releasing the pressure and opening and removing the mould. The mastic coating is applied up to the level of the concrete coating adjacent the joint.