PURPOSE: To easily prepare a heat exchanger made of aluminum excellent in pitting resistance by immersing a soldered heat exchanger made of aluminum in crosslinkable emulsion paint to which a solvent is added under heating to paint the heat exchanger in such a state that a resin bridge is not substantially formed between fins and subsequently crosslinking said paint.

CONSTITUTION: A heat exchanger made of aluminum soldered according to a usual vacuum soldering method is cooled and subsequently immersed in heated acrylic/melamine type emulsion resin paint, for example, to which isopropanol is added to be painted while the painted heat exchange is subjected to sag- cutting drying by air blow and heated at 170°C for 20min to crosslink the paint. Next, usual black acrylic/melamine type resin paint is applied to the heat exchanger by spraying and heated at 120°C for 20min to be crosslinked. By this method, excellent pitting resistance can be realized.