A method for obtaining a fibrous preparation for manufacturing a paper having significant water repellency and opacity in the dry or wet state. Fibres are suspended in water; calcium salts are added to the fibres; a source of carbonate salts in the form of an aqueous solution are added and the resulting mixture is stirred for a time sufficient to allow calcium carbonate particles to be precipitated. A solution of fatty acid salts heated to a temperature which corresponds to the micellisation temperature of said fatty acid salts is added and takes into account the critical micellar concentration of the fatty acid salts. The mixture is stirred thus obtained for a time sufficient to allow the fatty acids to be absorbed at the surface of the calcium carbonate particles and to obtain the desired fibrous preparation.