A method for continuously coating a substrate in motion such as a metal strip made of steel, the coating formed being an ultra-fine film of a thickness between 10 and 100 nm, deposited on the substrate: from a solution containing nanoparticles of oxides, in conditions of controlled pH, said substrate being at a temperature higher than 120° C., the total duration of the deposition being less than 5 seconds and preferably less than 1 second, characterized in that at least one chemical additive, called a “refiner”, is incorporated into said solution, said refiner having, mutatis mutandis, the effect of restricting the formation of said coating.