It depends on what type of paint that you want to use. The type of paint depends on how long you want it to last and how good you want it to look. If you want a professional grade paint job then look at Awlgrip LPU or Sterling LPU and follow the manufacturers directions for application.
If you want quick, cheap and not so good looking then roll or spray on a marine grade oil based paint. Look at Interlux 1 part enamel for this. They sell it at West Marine. I captain an aluminum boat for a living.