The old wall surface should be roughened and perfectly cleaned before plastering is commenced. It may be advisable to put the first coat on not thicker than 0.25 inch, and after this has set it may be cut and roughened by a pointing trowel. Then apply a second 1/4-inch coat and finish this to an even and smooth surface. Proportion of plaster: One-half part slaked lime, 1 part Portland cement, part fine, sharp sand, to be mixed well and applied instantly.