To provide a process for forming an improved coating film on a substrate, particularly to provide a coating film formed on a substrate, as adhered to the surface of the substrate, having a refractive index of from 1.28 to 1.38 and a contact angle with water of from 90° to 115°. A coating film having a refractive index of from 1.28 to 1.38 and a contact angle with water of from 90° to 115°, which is formed as adhered to a substrate surface by forming a reaction mixture comprising a silicon compound (A) of the formula Si(OR)4, a silicon compound (B) of the formula (R1O)3SiCH2CH2(CF2)nCH2CH2Si(OR1)3, an alcohol (C) of the formula R2CH2OH, and oxalic acid (D), in a specific ratio; heating this reaction mixture at a temperature of from 50 to 180° C. in the absence of water, to form a solution of a polysiloxane; then applying a coating fluid comprising the polysiloxane solution on a substrate surface to form a coating; and heat-curing the coating at a temperature of from 80 to 450° C.; a method for forming such a coating film; and a process for producing such a coating fluid.