There is described a method for applying an aqueous solution to the internal walls of a reactor, before the start of the polymerization reaction of vinyl chloride and/or vinyl acetate, which aqueous solution contains a salt of an anti-scaling agent which comprises a product of condensation of an aldehyde, a phenolic compound and an aromatic carboxylic acid hydroxylated at the aromatic nucleus, wherein the pH-value of this aqueous solution is changed, during the application, to a pH-value of less than 5 and, preferably, in the range between 2.5 and 4.5.