An improved method is disclosed for the soldering together of the cooling assembly of automobile radiators wherein aluminum fins, a first part of the cooling assembly, are etched in a solution consisting essentially of 10 to 14% sodium hydroxide, 4 to 7% zinc cyanide, 20 to 30% technical urea with the balance water for a period of 15 to 30 seconds at a temperature of 32 .degree. to 60.degree. C prior to undergoing treatment with a protective coating and eventual soldering to cooling pipes made of copper, brass, steel or aluminum, the remainder of the cooling assembly.