A device for moving a jet member in a first path over a layer to be treated by the jet has means configured to enable input of data relating to a geometrical shape of a surface of said layer to be treated by the jet member and means configured to use said data for calculating end positions (S1, S2) of an impact point of the jet member for a plurality of consecutive stripes (S) extending substantially perpendicularly to a second path for indexing, each having a width substantially corresponding to an indexing step and together forming said geometrical shape. A control arrangement co-ordinates movement of the impact point of the jet member and said indexing for treating a surface of said layer having said geometrical shape by said jet member.