NOVELTY - A reactive composition comprises isocyanate-reactive component and isocyanate functional component. Isocyanate-reactive component comprises short chain hydroxyl-functional compound (i) having two terminal hydroxyl groups, molecular weight of less than 250 g/mol and containing at least 2 carbon atoms, and high molecular weight hydroxyl-functional compound (ii) having two terminal hydroxyl groups and molecular weight of 250 g/mol or more. The isocyanate functional component is an isocyanate prepolymer (I).    USE - Reactive composition is used for forming polyurethane-based rain-erosion protective coating for rotor blades (claimed) of helicopter rotor and wind turbine for off-shore power station.    ADVANTAGE - The polyurethane-based rain-erosion protective coating formed using reactive composition has excellent smoothness.    DETAILED DESCRIPTION - A reactive composition comprises an isocyanate-reactive component and an isocyanate functional component. The isocyanate-reactive component comprises short chain hydroxyl-functional compound (i) having two terminal (( alpha )-( omega )) hydroxyl groups, molecular weight of less than 250 g/mol and containing at least 2 carbon atoms, and high molecular weight hydroxyl-functional compound (ii) having two terminal (( alpha )-( omega )) hydroxyl groups and molecular weight of 250 g/mol or more and comprising unit(s) chosen from oxyalkylene units and polyoxyalkylene units. The isocyanate functional component is an isocyanate prepolymer of formula: NCO-Z-NCO (I).    Z=linking group comprising at least two urethane (-NH-CO-O-) units and unit(s) chosen from alkylene, oxyalkylene, polyoxyalkylene, alkylene ester, oxyalkylene ester and/or polyoxyalkylene ester.    INDEPENDENT CLAIMS are included for the following:    (1) rain-erosion protective coating on external surface of rotor blade, which comprises reaction product of reactive composition comprising isocyanate-reactive component and isocyanate-functional component;    (2) method for protecting external surface of rotor blade from erosion, which involves applying reactive composition to the external surface of rotor blade and subjecting the reactive composition to curing;    (3) method of applying reactive composition for making protective coatings to a curved surface of article, which involves applying the reactive composition on the surface using a structured sheet which is structured to provide continuous path through which the composition can be distributed onto the surface. The sheet is bent around the curved surface and moved along the curved surface; and    (4) device for applying coatable substance onto curved surface. The device comprises structured sheet which is structured to provide continuous path through which coatable substance. The sheet is bent to fit around the curved surface to be coated, and the curved surface is rotor blade.