Abstract: Tanks for seagoing tankers for transporting liquefied natural gas or liquefied methone are basically prismatic, wherein the corners between intersecting walls are rounded off so that in such tanks adjacent flat wall parts are interconnected by a curved wall part. The invention relates to an apparatus and a method for mechanically treating the inner surfaces of the walls of such a tank, wherein the apparatus is so constructed that it is able to displace a treating tool along the flat wall parts and along the curved wall parts in such a manner that these wall parts are treated in a similar manner so that the quality of the treatment will be the same for all the various wall parts. The apparatus is particularly useful for spraying an insulating material, for example a polyurethane foam, onto the inner surface of the walls of such a tank.