Aquanamel vs Solagard. How much paint - 1 litre or 2 litres.

Got a small door/window painting job coming up, and I'm tossing up materials and quantity.
Some of these doors are going to get some sun so I need to paint them in a sun-proof material. And worst of all, my wife is insisting on black, which will probably be more affected by the sun. But as doors, they are also going to take some knocking so any paint needs to be tough.
So I'd like to know:
1. Should I use aquanamel or solaguard (or equivalents)?
2. Do I need 1 litre or 2 litres?


Any other advice on this?

Cheers.

.

but late for a reply, but for future visitors reference, a 100% acrylic gloss exterior is the best thing to use in this example, as black enamels, even water based will go chalky.
so your options are prepackaged black solagard or weathershield, or a dark tinted taubmans sunproof.