Method for reducing the roughness (Ra) of a ceramic thermal barrier coating with a defined coating composition and thermal expansion coefficient, said ceramic thermal barrier coating covering the surface of a Ni-, Co-, Fe-based superalloy component of a gas turbine to be thermally high loaded, wherein: - in a first step the ceramic thermal barrier coating is applied onto the surface of a component by atmospheric plasma spraying and the roughness (Ra) of said ceramic thermal barrier coating is measured and - in a following step a zirconia based slurry coating with alumina silicate or zirconium silicate additives and a slurry coating binder being a silicate solution, phosphate solution or silicon emulsion is prepared for applying onto the surface of the ceramic thermal barrier coating, the coating composition of the zirconia based slurry coating being tailored to have a similar thermal expansion coefficient like the ceramic thermal barrier coating; - calculating a minimum number of coating spray passes necessary to provide the zirconia based slurry coating with a thickness that is at least two times of the roughness (Ra) of the ceramic thermal barrier coating, - applying the zirconia based slurry coating with said calculated number of spray passes onto the surface of the ceramic thermal barrier coating; - fully curing, but only partially sintering the zirconia based slurry coating at a temperature in the range of 300 to 800°C; - polishing the zirconia based slurry coating; - completely sintering the zirconia based coating only after the first exposition to thermal loaded operation of the component; the polishing step is performed until the peaks of the roughness (Ra) are reached so the thickness of the zirconia based slurry coating becomes a reduced thickness (T') substantially equal to the roughness (Ra) of the ceramic thermal barrier coating such that finally the zirconia based slurry coating does cover the ceramic thermal barrier coating only locally at the valleys of the roughness (Ra). - in a first step the ceramic thermal barrier coating is applied onto the surface of a component by atmospheric plasma spraying and the roughness (Ra) of said ceramic thermal barrier coating is measured and - in a following step a zirconia based slurry coating with alumina silicate or zirconium silicate additives and a slurry coating binder being a silicate solution, phosphate solution or silicon emulsion is prepared for applying onto the surface of the ceramic thermal barrier coating, the coating composition of the zirconia based slurry coating being tailored to have a similar thermal expansion coefficient like the ceramic thermal barrier coating; - calculating a minimum number of coating spray passes necessary to provide the zirconia based slurry coating with a thickness that is at least two times of the roughness (Ra) of the ceramic thermal barrier coating, - applying the zirconia based slurry coating with said calculated number of spray passes onto the surface of the ceramic thermal barrier coating; - fully curing, but only partially sintering the zirconia based slurry coating at a temperature in the range of 300 to 800°C; - polishing the zirconia based slurry coating; - completely sintering the zirconia based coating only after the first exposition to thermal loaded operation of the component;