To prepare a galvanized roof for painting:
1. Insure that the roof is clean, preferably by pressure washing to remove, dirt, pollen, and tannic acid (if there are oak or pine trees nearby).
2. Scrape or sand to remove any loose rust or peeling paint.
3. Prime any bare non-rusty galvanized metal with a 100% acrylic primer (this can be tinted to your finish color or to a gray to go under aluminum paint).
4. Prime any rusty areas with a rust-inhibitive oil-base primer (do not use a water-based product to prime rust) and allow to dry overnight.
5. After following the previous steps you may top coat with either an oil or water based paint.