FIELD: construction material industry. SUBSTANCE: method involves adding to powder plastering at least one component A comprising at least one alkyl hydropolysiloxane and at least one hydrocolloid B comprising at least one galactomannan. Powder composition comprising hydrocolloid B is solid substance based on at least one component A applied to carrier. Molded product is made in the form of sheet or plate manufactured from plastering subjected to hydrophobization and containing component A comprising at least one alkyl hydropolysiloxane and component B comprising at least one galactomannan. Method imparts water tightness to plastering without changing its mechanical properties. EFFECT: simplified method and improved quality of plastering. 20 cl, 2 tbl, 2 ex
