Use the recomended primer for the paint, and be sure it is for use on properly prepared aluminum. It doesn't have to be Zinc Chromate. This will normally be a 2-part primer for Urethanes (Urethanes are only one option, BTW). There are 2-part Zinc Chromate primers available for use with some Urethanes, but may not be compatible for all. You're looking for general answers, but you'll have to do specific research on each individual product you intend to use, to ensure compatability, before settling on all of the variables involved.