The present invention refers to a process for the preparation of an additive with a metallic hydroxide base, and especially of magnesium hydroxide, to be integrated in coatings with the purpose to give them higher flame retarding properties. The hydroxide has an average particle size that is selected from between 1 nanometer and 10 microns preferable with a wide variety, the magnesium hydroxide is submitted to a treatment of washing and dispersion, mainly so that they efficiently are dispersed in, and do not interfere with, the desired properties of the coating. The nature of the coating function is selected from the materials and the conditions of the treatment, as well as the size of the particle. The objective coating can have a base of water, solvents, oil, and alcohol. Compared with coatings formed with other flame retarding compositions, the additive of the invention presents less loss of weight by means of burning through ASTM D1360 standards.