The ratio of cement and sand for plastering is given below. 6 MM thickness of cement plaster and cement mortar 1:3 or 1:4 is recommended for cement plastering on RCC surfaces, on the RCC walls, Fascia etc. or as directed by the Engineer, 6 MM to 10 MM thickness of cement plaster and cement mortar in 1:3 or 1:4 ratios is recommended for underside of RCC slabs. 12 MM thick cement mortar in 1:4 ratios is recommended on the walls where smooth side exists. Either 9 or 4.5 MM thick and 15 MM thick cement plaster in 1:4 ratios is recommended on the rough side of the walls. 20 MM thick cement mortar in 1:3 with two coats is recommended in case of vertical Damp Proof Course (DPC). 6 MM thick cement mortar in 1:3 with a coat of white washing is recommended in case of bearing plaster on the walls under the slab. 15 MM thick cement mortar in 1:3 is used with 3 MM thick cement rendering on plaster in case of Dado.Ready Plaster :An appropriate ratio if you want to use lime would be six parts of sand to two parts of lime to one part of cement.Sand: Lime: Cement::6;2;1