Sol for the sol-gel coating of a surface, said sol comprising, in percent by weight: a)—3% to 30%, preferably 5% to 20%, more preferably 7% to 15%, especially 8% to 14%, more especially 10% to 13%, for example 10.8% or 12%, of at least one organometallic compound of zirconium, aluminium or titanium; b)—5% to 50%, preferably 5% to 40%, more preferably 10% to 40%, especially 15% or 20% to 30%, for example 22% or 23%, of at least one organosilane compound; c)—1% to 15%, preferably 2% to 10%, more preferably 3% to 8%, for example 5%, of at least one compound selected from acids, bases, glycols and ethoxyethanol; d)—the remainder to 100% of demineralized or distilled water; the total amount of a) and b) being greater than 30%, preferably greater than 31.2%; 31.5%; 32%; or 33%, more preferably greater than 35%, especially greater than 40%, more especially greater than 50%. Kit comprising: a first container containing a first part A, in liquid form, comprising components a), c) and d) of the sol; a second container containing a second part B, in liquid form, comprising component b) of the sol. Process for preparing a sol-gel layer employing said sol and layer thus obtained, and substrate coated with at least one such sol-gel layer.Process for preparing a coating comprising two or more layers on a surface of a substrate, at least one of these layers being a sol-gel layer prepared by the process above.