The paint cannot be glossy on the face of the dish.
I used flat-grey primer, and applied two or three light coats.
Works fine.
Make sure the surfaces are clean.
You might want to lightly rough up the old paint with a Scotch-Brite pad (my favorite), but don't go crazy and rub through it.
The old paint will be your undercoat, so normally there should be no priming required.
edit:
.... uhhhhh..... what 'brand new' dish do you have that's aluminum?
None come to mind, right off.
We talking Ku dish or a BUD???
AND, one last thought.
If you do put on some unusual color or artwork like Doug did above, and use glossy paints,...
Just do a final layer of flat clear, and you'll be good to go.