Abstract: The solution covered by the invention contains 0.5 to 9.0% by mass of hydrate salt of chloride anion Cl+ with a chemical element of bivalent kation of cobalt or copper, magnesium, manganese, zinc, calcium or a monovalent kation of sodium, potassium; 0.5 to 10.5% by volume of oxalic acid, formic acid, acetic acid, butyric acid, citric acid, lactic acid, hydrofluoric acid, boric acid, tartaric acid; 0.2 to 8.0% by mass of casein and 80.5 to 98.5% by volume of water. The overall pH of the solution is 0.8 to 3.5%.