Yes, paint won't stick to aluminum unless you prime it first with a special primer. I forget what the primer is called, but it bonds with the aluminum and sets base for paint. Normal primer nor paint will stick to aluminum and will flake off over a period of time.
Go to an auto paint shop and they should be able to tell you what the name of that stuff is... BTW, you said you were cheap.. that primer is rather expense... somewhere around $30/quart if I remember correctly... Ran across all this when I was painting an aluminum boat for a friend...