A method of grinding an inner surface of a cylindrical tank, said cylindrical tank comprising a convex first part, a convex second part and a cylindrical part interconnecting said convex first part and said convex second part, said convex first part, said convex second part and said cylindrical part together defining said inner surface, said cylindrical part defining a central axis, said method comprising the step of providing a grinding assembly, said assembly comprising: a frame for holding said cylindrical tank and preventing any substantial movement of said cylindrical tank except around said central axis, said frame comprising a motor and a drive mechanism connected to said motor, said drive mechanism being capable of interacting with said cylindrical tank, and a stand rotably connected to said frame, said stand defining a tilt axis in relation to said frame, said tilt axis being horizontal and perpendicular to said central axis, said method comprising the additional steps of: holding said cylindrical tank by said frame and causing said frame to assume a horizontal position in which said convex first part and said convex second part are level, placing an amount of grinding material into said tank such that said grinding material covers a part of said inner surface and extends between said convex first part and said convex second part, rotating said cylindrical tank about said central axis by using said motor and said drive mechanism at a rotational speed of less than 100rpm and causing said grinding material to move in a sliding and non-tumbling movement relative to said inner surface, causing said frame to assume a first tilted position in which said convex first part is located above said convex second part and said grinding material extending on said inner surface between said cylindrical part and a first point of interception between said convex second part and said central axis, rotating said cylindrical tank about said central axis by using said motor and said drive mechanism at a rotational speed of less than 100rpm and causing said grinding material to move in a sliding and non-tumbling movement relative to said inner surface, causing said frame to assume a second tilted position in which said convex first part is located below said convex second part and said grinding material extending on said inner surface between said cylindrical part and a second point of interception between said convex first part and said central axis, rotating said cylindrical tank about said central axis by using said motor and said drive mechanism at a rotational speed of less than 100rpm and causing said grinding material to move in a sliding and non-tumbling movement relative to said inner surface. a frame for holding said cylindrical tank and preventing any substantial movement of said cylindrical tank except around said central axis, said frame comprising a motor and a drive mechanism connected to said motor, said drive mechanism being capable of interacting with said cylindrical tank, and a stand rotably connected to said frame, said stand defining a tilt axis in relation to said frame, said tilt axis being horizontal and perpendicular to said central axis, holding said cylindrical tank by said frame and causing said frame to assume a horizontal position in which said convex first part and said convex second part are level, placing an amount of grinding material into said tank such that said grinding material covers a part of said inner surface and extends between said convex first part and said convex second part, rotating said cylindrical tank about said central axis by using said motor and said drive mechanism at a rotational speed of less than 100rpm and causing said grinding material to move in a sliding and non-tumbling movement relative to said inner surface, causing said frame to assume a first tilted position in which said convex first part is located above said convex second part and said grinding material extending on said inner surface between said cylindrical part and a first point of interception between said convex second part and said central axis, rotating said cylindrical tank about said central axis by using said motor and said drive mechanism at a rotational speed of less than 100rpm and causing said grinding material to move in a sliding and non-tumbling movement relative to said inner surface, causing said frame to assume a second tilted position in which said convex first part is located below said convex second part and said grinding material extending on said inner surface between said cylindrical part and a second point of interception between said convex first part and said central axis, rotating said cylindrical tank about said central axis by using said motor and said drive mechanism at a rotational speed of less than 100rpm and causing said grinding material to move in a sliding and non-tumbling movement relative to said inner surface.