SDI recommends type A, both A40 and A60, over type G due to the superior primer adhesion qualities. A60 is superior to A40 for rust inhibiting.
SDI Technical Data Series 100 has been republished as an ANSI Standard, A250.8.In addition, please see our Technical Data Series 112 "Zinc-Coated Standard Steel Doors and Frames." The difference in the A and G designations are explained. Table 1 of that document gives coding designations for A40, A60, G60 and G40.