jeff,
is this spray enamel or brush on? do you bake each coat on or did you apply the three coats then bake it? i have some old 105 cranks i might give this a try on... i'm not too sure about the heat treat of cast aluminum either, but with welding 6000 series aluminum it needs to be heat treated by baking and quenching in oil, where as 7000 series aluminum is baked for awhile around 400ish degrees. so perhaps the 120 degrees wouldn't affect be hot enough to affect it...