PURPOSE: To eliminate the generation of stagnation of paint and prevent the cracking cf a coated film after drying and setting by providing fine grooves near the heat transfer tube penetrating hole of a fin.

CONSTITUTION: Heat transfer tube penetrating holes are bored on a plate fin 1 and fin collars 2 are raised up from the fin 1 with an equal interval while the heat transfer tubes 3 are fixed to the plate fin 1 by expanding the tubes after penetration to contact the pipes with the fins closely and the plate fin 1 is provided with fine grooves 7 near the heat transfer tube penetrating holes. Accordingly, coating 8 is stagnated once in the upper part of the heat transfer tubes of a heat exchanger, dipped into functional coating to coat it and pulled up from a dip tank, while excessive coating 9 flows along the fine grooves 7 near the heat transfer tube penetrating holes and arrives at the end face of the fin, then, drops down by the gravity of the same. According to this method, excessive coating on the surface of the fin will never be stagnated at the lower part of the heat transfer tubes and the generation of cracking of a coating film is eliminated after the coating is dried and set whereby the corrosion of a heat exchanger may be prevented.