A coating system for in-situ coating of a surface formed by a plurality of plasterboard sections comprises a dry powder composition which is dispersible in an aqueous applicator medium and provides a very high quality surface finish to which gloss or matt paint may be applied directly without joint photographing or show-through of fasteners. The coating composition may be applied by a spray coating technique or a roller technique. The dry powder composition comprises calcium carbonate and, by weight, relative to calcium carbonate, from 4% to 11% of a polymeric binder, from 0.60% to 1.5% of a thickening agent and from 0.10% to 0.35% of a surfactant.