I think that the best base for painting a zamac car is the original paint ,I dont know how they paint them but the paint is very resistant, the only problem I can see with this is that sometimes the final result is a thick layer of paint; just as AOS says just rough the original paint with a 1200 sand paper, and then apply a layer of primer this will prevent the green edges appear after painting the final color layer
If you choose to peal the paint, you must use a primer layer after applying any color to the body, this will prevent the paint cracks and peel in the future