A method of fabricating a heat exchanger which includes: obtaining multiple coolant-carrying conduit sections, each having a non-circular cross-section, and first and second main surfaces; providing multiple primary folded fins secured to the conduit sections. Each folded fin includes a solid fin surface with multiple bends defining alternating, U-shaped air-passage channels, and a base surface and a top surface. Each folded fin is secured at the base or top surface to a main surface of a conduit section, and the folded fins have leading and trailing edges relative to airflow direction. The method includes forming a plurality of sets of secondary fins, each set extending from the leading or trailing edge of a respective folded fin at an angle other than 0. At least one conduit section has a first folded fin secured to its first main surface, and a second folded fin secured to its second main surface thereof.