tetraalkoxysilane represented by the general formula Si(OR3)4 where R3 is an alkyl group, (b) from 5 to 40% by weight of a fluoroalkylalkoxysilane represented by the general formula R4 CH2 CH2 Si(R5)a (OR6)3-a where R4 is a perfluoroalkyl group, R5 is a monovalent hydrocarbon group, R6 is an alkyl group and a is an integer of 0, 1 or 2, and (c) from 3 to 20% by weight of a diorganopolysiloxane te