I have a black Lexus and a dark green Toyota and Zaino makes both look great. I have applied Zaino to several not-new cars, and one new car (the black Lexus). I would suggest the following:
Dawn wash, then use some mild polishing product to remove any oxidized paint and as much of the surface paint marks as possible. Then clay, and wash again to remove the clay.
Now start with Zaino. In the old days, we had to start with Z1, and then either Z2 or Z5, and a second coat could not be applied for another 24 hours. However, you really don't get the shine going until at least 2 or three coats. However, if you get the ZFX additive, it cures instantly and you can apply as many as 3 coats in immediate succession. What I do is mix the ZFX with Z5 (no need for the z1) and apply two coats of that, and then use Z2 for the third coat.
For subsequent applications, you can add one coat and skip the ZFX, or use it and apply 2 or 3 in the same day. As you probably know, this does not take a lot of time and is not hard work, as you don't have to buff.
A coat or two of Zaino every 3 to six months will keep it looking like new.